,%m  UC-NRLF 


Manipulation 
of  the  Microscope 


EDWARD  BAUSCH 


LIBRARY 


UNIVERSITY  OF  CALIFORNIA. 
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A«m.  INK-    Deceived  J>tt/L^L£-^         ,  i8g  b 

^Accessions  No.     //&>  ~  QMS  No. 


MANIPULATION 


.  OF    THK 


MICROSCOPE 


BY 

EDWARD    BAUSCH 


THIRD  EDITION 
FIFTEENTH  THOUSAND 


PUBLISHED    BY 

BAUSCH    &    LOME    OPTICAL    COMPANY, 
ROCHESTER,    N.  Y. 


GA\ 


Aim.  Bit* 


. 

Copyright 
KAI'SCII  &  LOMB  OTTICAL  COMPANY 


PREFACE  TO  FIRST  EDITION. 

It  may  seem  to  some  persons  an  act  of  presump- 
tion for  a  maker  of  microscopes  and  microscopic 
accessories  to  enter  the  field  of  authorship  and 
attempt  to  supplement  the  valuable  labors  which 
in  recent  years  have  made  the  use  of  the  micro- 
scope an  indispensible  aid  in  the  advancement  of 
science. 

To  such,  if  any,  I  submit,  that  being  a  producer 
of  microscopes  and  their  accessories,  I  have  had 
opportunity  to  become  acquainted  with  the  lack  of 
general  knowledge  of  the  fundamental  principles 
of  the  instrument  and  the.best  method  of  technique, 
even  among  owners  of  microscopes.  Indeed,  with 
so  many  complications,  with  almost  unlimited  pow- 
ers and  uses  of  the  instrument,  the  beginner  can- 
not fail  to  feel  the  need  of  a  guide  and  adviser. 

In  order  to  accomplish  the  greatest  good,  I  have 
started  out  in  this  little  Manual  with  the  supposi- 
tion that  the  purchaser,  or  owner,  is  a  beginner, 
and  absolutely  ignorant  of  the  microscope  and 
everything  which  pertains  to  it,  and  therefore  have 
attempted  to  convey,  step  by  step,  in  as  simple 
language  as  I  could  command,  information  which 
will,  I  trust,  lead  to  ease  of  manipulation  and  give 
both  pleasure  and  profit  to  those  for  whom  it  was 
specially  written. 

With  these,  its  purposes  and  hopes,  I  beg  for  my 
self-imposed  labor  a  friendly  reception. 

EDWARD  BAUSCH. 


OF 

UNIVERSITY 


PREFACE  TO  SECOND  EDITION. 

The  demand  for  this  book  having  considerably 
exceeded  the  expectations  of  its  author  and  the 
comments  on  its  utility  having  been  so  favorable 
leads  to  the  view  that  it  fills  a  gap  in  microscopical 
literature. 

In  preparing  for  a  new  edition  an  opportunity 
has  been  given  for  enlarging  on  some  of  the  sub- 
jects and  rewriting  others,  so  as  to  make  them 
conform  to  the  changes  which  the  last  five  years 
have  brought  about  in  the  construction  of 
apparatus. 

While  it  may  be  true  that  many  of  the  subjects 
might  be  treated  much  more  extensively,  the  writer 
has  purposely  refrained  from  doing  so,  because  he 
has  considered  it  beyond  the  province  of  his  inten- 
tion and  because  books  giving  more  extensive 
information  are  available. 

An  intending  purchaser  of  a  microscope  finds  it 
more  or  less  difficult  to  make  a  suitable  selection 
and  while  it  is  always  best  to  consult  an  experi- 
enced microscopist,  the  writer  has  endeavored  to 
convey  information  which,  he  hopes,  will  aid  in 
this  direction. 

THE  AUTHOR. 
May,  1891. 


PREFACE  TO  THIRD  EDITION. 


The  past  demand  for  this  little  volume  makes 
extended  remarks  superfluous,  the  new  edition 
appearing  as  evidence  that  it  is  considered  of  some 
value. 

This  edition  has  been  almost  entirely  rewritten 
to  bring  it  in  accord  with  the  advance  which  has 
been  made  in  the  construction  of  microscopes  and 
accessories  and  while  it  is  not  expected  to  be  a 
complete  guide,  it  is,  nevertheless,  hoped  that  it 
will  make  the  labor  of  the  beginner  more  easy. 

Since  its  first  issue  there  have  appeared  two 
books  covering  the  same  purpose:  "  The  Micro- 
scope and  Microscopical  Methods  "  by  Prof.  S.  H. 
Gage  of  Cornell  University  and  "  Microscopical 
Praxis  "  or  "  Simple  Methods  of  Ascertaining  the 
Properties  of  Microscopical  Accessories "  by  Dr. 
A.  C.  vStokes,  both  of  which  are  heartily  commended 
to  the  microscopist.  Neither  should  be  wanting-  in 
a  microscopical  library.  The  writer  is  also  pleased 
to  acknowledge  the  suggestions  of  an  enlarged 
scope  for  this  book,  which  he  has  obtained  by  a 
perusal  of  them,  as  well  as  from  the  admirable 
work  of  Dr.  W.  H.  Dallinger  in  the  latest  issue  of 
Carpenter,  "The  Microscope  and  Its  Revelations" 


which  may  be  commended  to  those  who  wish  to 
study  more  deeply  the  principles  of  the  microscope 
and  learn  its  history  and  development. 

The  writer  trusts  that  omissions  will  be  pardoned, 
as  the  only  time  which  it  has  been  possible  to 
devote  to  this  work  has  been  the  spare  moments  of 
a  busy  life  and  hopes  sufficient  information  may 
be  obtained  to  give  full  compensation  for  such 
defects. 

THE  AUTHOR. 
March  1,  1897. 


CONTENTS. 


Purpose  of  the  Microscope;  Simple  and  Compound  Micro- 
scopes; Lenses;  To  Determine  the  Focus  of  a  Lens; 
Magnifying  Power;  Spherical  Aberration;  Chromatic 
Aberration. 

SIMPLE  MICROSCOPES. 

Coddington  Lenses;  Aplanatic  Triplet;  Hastings'  Triplet; 
Reading  Glass;  Bruecke  Lens;  Stands  and  Holders; 
How  to  Use  Magnifiers  and  Dissecting  Microscopes; 
Magnifying  Power;  To  Determine  Magnifying  Power. 

THE  COMPOUND  MICROSCOPE. 

Description  of  Parts;  A,  Base;  B,  Pillar;  C,  Arm;  D,  Body; 
E,  Nose-Piece;  Society  Screw;  F,  Objective;  G,  Eyepiece 
or  Ocular;  H,  Draw-Tube;  I,  Collar;  J,  Coarse  Adjust- 
ment; K,  Milled  Heads;  L,  Fine  Adjustment;  M,  Stage; 
N,  Clips;  Centering  Screws;  O,  Mirror;  P,  Mirror-Bar; 
Q,  Substage;  R,  Substage  Bar;  S,  Diaphragm;  Optical 
Axis;  Object;  Slide  or  Slip;  Cover  Glass;  Classification 
of  Microscopes;  Tube  Length;  Stage;  Revolving  Stage; 
Glass  Stage;  Mechanical  Stage;  Nose-Piece;  Double 
Nose-Piece;  Bodies  or  Tubes;  Coarse  Adjustment;  Fine 
Adjustment;  Draw-Tube;  Base;  Joint  for  Inclination; 
Mirror  and  Mirror-Bar;  Diaphragm. 


OBJECTIVES  AND  EYEPIECES. 

Objectives;  Tube  Length;  Nomenclature,  or  Rating  of 
Objectives;  Powers;  Systems;  Angular  Aperture ;  Cover 
Glass;  How  to  Measure  Angular  Aperture;  Resolving 
Power;  Chromatic  Aberration ;  Spherical  Aberration  and 
Cover  Glass;  Penetration;  Flatness  of  Field;  Working- 
Distance;  To  Measure  Working  Distance;  Magnifying 
Po\ver;  Apochromatic  Objectives;  Eyepiece  or  Ocular; 
Huyghenian  Eyepiece;  Solid  Eyepiece;  Ramsden  Eye- 
piece; Periscopic,  Orthoscopic  and  Kellner  Eyepieces; 
Compensating  Eyepiece;  Rating  or  Designation;  Flat- 
ness of  Field;  Size  of  Field;  Defects. 

HOW  TO  WORK. 

To  Attach  Eyepiece;  To  Attach  Objective;  Finding  an 
Object;  To  Illuminate  the  Object;  How  to  Focus;  Which 
Eye  to  Use;  What  Objects  to  Use;  Test  Plate;  How  to 
Work;  Opaque  Object;  Medium  Power  of  Objective; 
Chromatic  Aberrations;  Cover  Glass;  Dry  Adjustable 
Objectives;  Immersion  Objectives;  Immersion.  Objec- 
tives on  Test  Plate. 

ILLUMINATION  WITH  CONDENSER. 

Purpose  of  the  Condenser;  Abbe  Condenser;  Centering; 
Centering  the  Illumination;  To  Focus  Condenser;  Rela- 
tion of  Aperture  of  Condenser  to  Objective;  Oblique 
Light  with  Condenser. 


HOW  TO  DRAW  OBJECTS. 

To  Determine  Magnifying  Power;  To  Measure  the  Size  of 
an  Object. 

TO  SELECT  A  MICROSCOPE. 

Stand,  American  or  Continental;  Tube  Length;  Base;  The 
Joint  for  Inclination  of  Arm;  Coarse  Adjustment;  Rack 
and  Pinion;  Fine  Adjustment;  Metal;  Size  and  Weight; 
Working  Space  Below  Stage;  Stage;  A  Glass  Stage  and 
Slide  Carrier;  The  Mechanical  Stage;  Revolving  Stage; 
Substage;  Substage  Condenser;  Objectives  and  Eye- 
pieces; Eyepiece;  Objectives;  Objectives  of  Wide 
Aperture;  Accessories. 

CARE  OF  A  MICROSCOPE. 

To  Take  Care  of  a  Stand;  To  Take  Care  of  Objectives  and 
Eyepieces. 


INTRODUCTORY. 

The  knowledge  which  this  volume  attempts  to 
convey  in  the  proper  use  of  the  microscope,  may 
be  gleaned  by  following  several  methods. 

First. — By  reading  carefully  all  contents  from 
first  to  last.  This  is  strongly  recommended,  as  an 
effort  has  been  made  to  make  the  contents  progres- 
sive and  it  is  hoped  that  in  this  manner  a  general 
knowledge  of  terms  and  optical  principles  involved 
will  be  acquired,  which  will  greatly  aid  in  intelli- 
gently and  with  greater  facility  and  pleasure 
carrying  out  the  instructions  in  the  chapter  "  How 
to  Work." 

Second. — By  reading  the  general  principles  and 
studying  the  various  terms  and  omitting  some  of  the 
instructive  processes,  as  "  How  to  Judge  Chromatic 
and  Spherical  Aberration,"  "  How  to  Measure 
Angle,"  "  How  to  Measure  Working  Distance," 
etc.,  which  may  be  carried  out  after  some  famili- 
arity with  manipulation  of  the  instrument  has  been 
acquired. 

Third. — By  studying  the  parts  of  the  stand  and 
then  referring  to  chapter  "  How  to  Work."  This 
is  not  in  any  case  advised,  but  may  be  done  when 


14 

there  is  a  pressing-  requirement  for  the  use  of  the 
instrument,  or  great  anxiety  to  operate  it.  The 
previous  chapters  should  then  be  studied  later  on. 

Fourth. — If  the  reader  does  not  possess  a  micro- 
scope and  desires  to  purchase  one,  study  the  parts 
of  the  stand,  general  principles  and  "  How  to 
vSelect  a  Microscope." 


OPTICAL  PROPERTIES  OF  LENSES. 


Purpose  of  the  Microscope. — The  microscope 
is  an  instrument  which  magnifies  near  objects,  so 
that  we  are  better  able  to  examine  their  structure 
than  is  possible  with  unassisted  vision. 

Simple  and  Compound  Microscopes. — Micro- 
scopes are  divided  into  two  classes — simple  and 
compound — the  difference  between  the  two  being 
as  follows:  With  the  simple  microscope  the  object 
is  viewed  directly  and  the  magnified  image  shows 
the  object  erect,  or  in  its  real  position.  It  must 
consist  of  one  lens,  but  may  consist  of  more,  which 
however  are  in  close  contact.  With  the  compound 
microscope  a  magnified  image  is  observed  and  this 
in  a  reversed  position  from  the  true  one  of  the 
object,  so  that  what  is  right  in  the  object  is  left  in 
the  image.  This  must  consist  of  at  least  two 
lenses  with  suitable  distance  between  them. 

Lenses. — As  microscopes  depend  upon  the 
action  of  lenses  it  seems  fitting  that  these,  as 
well  as  the  action  of  light  passing  through  them, 


16 

should  receive  attention.  Every  person  has  un- 
questionably observed  that  when  a  spoon  is  placed 
in  a  tumbler  of  water  it  is  apparently  bent  at  the 
surface  of  the  water,  or  when  looking  at  an  object 
lying  in  the  bottom  of  a  dish,  it  is  apparently  at  a 
different  point  than  when  viewed  from  the  side. 
This  is  caused  by  the  great  principle  of  actually 
bending  the  rays  of  light  as  they  pass  from  one 
transparent  medium  into  another  of  greater  or  less 
density  and  is  called  refraction.  The  amount  of 
refraction  increases  as  the  difference  in  the  density 
of  the  two  media  becomes  greater.  We  also  know 
that  in  viewing  objects  through  a  glass  prism,  they 
apparently  lie  in  a  different  direction  than  their 
real  one.  The  amount  of  this  deviation  is  absolute 
and  depends  for  the  one  factor  upon  the  density  of 
the  glass  composing  the  prism  and  for  the  other 
upon  its  shape. 


Fig.  i. 

Fig.  1  represents  a  cross  section  of  a  prism 
and  shows  how  the  ray,  as  it  touches  the  first 
surface  of  the  prism,  undergoes  refraction  and  on 


17 

its  emergence  from  the  prism,  its  second  refraction 
takes  place;  it  will  also  be  noticed  that  the  light 
is  bent  downward  or  toward  the  base  of  the  prism 
and  if  the  prism  be  imagined  reversed  with  its  base 
upward,  the  action  of  the  prism  on  the  light  will 
be  the  same,  but  its  course  will  be  upward,  always 
toivard  its  base. 


Now  a  lens  in  either  of  its  two  principle  forms  is 
nothing  more  in  effect  than  the  prisms  showing 
as  in  Fig.  2,  where  the  two  bases  are  placed 


together,  how  the  light  is  refracted  toward  the 
bases  and  thus  converges;  or  how,  as  in  Fig.  3, 
where  the  bases  are  above  and  below,  the  action  of 


18 

the  prisms  on  the  light  still  remains  the  same, 
refracting  the  rays  of  light  toward  the  bases,  and 
in  this  case  causing  them  to  diverge  or  separate. 

If  the  combinations  of  prisms  be  imagined  with 
curved  surfaces  instead  of  flat  ones,  the  action  of 
light  through  them  will  be  precisely  the  same  and 
we  have  the  two  great  classes  of  lenses,  converging 
or  convex  and  diverging  or  concave,  as  shown 
in  Fig.  4. 


Fig.  4. 

These    two   classes    are    again    subdivided,    as 
shown  in  Fig.  5,  showing  the  three  forms  which 


Fig.  5. 

are  made  in  each  of  the  convex  and  concave  types 
and  which  are  called  in  rotation' as  drawn,  double 


convex ',  plane  convex,  convex  meniscus  and  double 
concave,  plane  concave,  concave  meniscus. 

In  the  convex  lenses,  parallel  rays  are  converged 
to  one   point  as  shown  in    Fig.  6,  in  which  c  is 


Fig.  6. 

the  principal  focus,  or  focal  point  and  the  distance 
from  b  to  c  the  focal  distance. ,  With  the  same 
lens,  if  a  flame  be  placed  at  c,  all  the  rays  which 
strike  the  lens  will  be  emitted  in  a  parallel  direc- 
tion on  the  opposite  side.  The  straight  line  d  b 
c  which  passes  through  the  middle  of  the  lens  is 
called  the  principal  axis  and  the  line  a  c'  the 
radius. 

The  radius  determines  the  power  or  converging 
quality  and  as  it  becomes  longer,  reduces  the 
power  by  making  the  lens  longer  focus.  If  in  a 
double  convex  lens  the  radius  of  each  surface  is 
1  inch,  the  lens  has  a  focus  of  1  inch  and  if 
in  a  plane  convex  lens,  the  convex  surface  has  the 
same  radius,  the  focus  is  twice  as  long,  or  2  inches. 

In  a  concave  lens  the  action  of  the  lens  is  oppo- 


so 

site;  instead  of  converging  the  light  toward  the 
axis,  it  diverges  the  light  from  the  axis  as  shown 
in  Fig.  7.  The  imaginary  extension  of  the 

,£• 


diverging  rays  should  meet  at  c  and  the  distance 
e  d  indicates  the  virtual  focus,  in  contradistinction 
to  the  real  focus  in  the  convex  lens. 

To  Determine   the   Focus  of  a   Lens. — The 

focus  of  a  lens  may  be  quite  accurately  determined 
by  the  following  methods:  Taking  the  sun  as  an 
object,  hold  the  lens  toward  it  and  bringing  a  piece 
of  paper  under  it,  move  it  slowly  toward  the  lens. 
A  large  bright  spot  will  appear,  which,  as  the 
paper  is  brought  nearer,  will  decrease  in  size  but 
increase  in  intensity  until  it  becomes  quite  minute 
and  as  the  paper  is  brought  still  closer  to  the  lens, 


21 

the  image  will  be  found  to  enlarge  again.  Return 
until  the  most  intense  point  is  reached  again.  By 
holding  sufficiently  long,  especially  if  the  paper  be 
dark,  it  will  be  found  to  burn,  due  to  the  concen- 
tration of  heat  rays  and  hence  is  called  the  burn- 
ing point  vr  focal  point.  Or,  in  a  room  opposite  a 
window  or  lamp,  hold  a  rule  against  a  white  wall 
and  placing  the  edge  of  a  lens  upon  it,  move  it 
slowly  toward  the  wall  until  a  greatly  reduced,  but 
bright  image  of  either  object  appears  and  read  off 
the  distance  between  the  wall  and  edge  of  lens, 
which  is  its  focus. 

A — ^_ 


i 
A 

/'\ 
A 


Fig.  8. 

Magnifying  Power. — Magnifying  power  of  a 
convex  lens  depends  also  upon  its  convergence. 
In  Fig.  8  a  b  represents  the  object,  c  d  the 
lens,  e  the  pupil  of  the  eye.  By  following  the 


22 

course  of  rays  from  the  object  it  will  be  noted  how 
they  are  converged  and  refracted  by  the  lens  and 
intercepted  by  the  pup.il  of  the  eye.  If  now  the 
lines  between  c  and  e  be  considered  elongated, 
they  will  be  found  to  meet  beyond  a  b  and  there 
form  a  virtual  image.  It  will  be  well  to  under- 
stand at  this  point  the  difference  between  a  real 
and  a  virtual  image.  The-raz/  image  is  one  which 
can  be  accurately  seen  and  projected  upon  a  sur- 
face, as  in  the  magic  lantern,  or  in  the  picture 
which  is  given  by  the  photographic  camera.  The 
virtual  image  cannot  be  so  shown.  In  a  lens 
of  less  convexity  or  longer  focus,  there  is  less 
convergence  of  rays  and  the  length  of  the  virtual 
image  is  consequently  reduced  and  thus  the  mag- 
nification is  less. 

Spherical  Aberration.  —  In  considering  the 
refraction  of  light  by  a  lens  up  to  this  point,  we 
have  purposely  avoided  mentioning  another  quality 
which  is  incident  to  it.  In  magnifying  an  object 
by  a  single  lens  it  will  be  noticed  that  the  virtual 
image  as  seen  through  its  central  portion  is  quite 
clear,  while  that  near  the  margin  or  edge  is  quite 
indistinct.  This  is  due  to  spherical  aberration  and 
the  extent  of  this  aberration  increases  with  the 
power  of  the  lens.  It  is  due  to  the  unequal  refrac- 
tion between  those  rays  passing  near  the  margin 
and  those  passing  through  the  central  portion,  so 
that  the  rays  instead  of  combining  at  the  focal 


point,  come  tog-ether  at  different  intervals  along 
the  central  line  .or  axis.  By  reference  to  Fig.  i) 
it  will  be  seen  that  the  outer,  or  marginal  rays  are 


refracted  at  c  and  f  so  that  they  will  combine  at 
g  and  the  inner,  or  central  rays  are  refracted  at 
j  and  k  so  that  they  will  meet  at  /.  In  the  same 
manner  will  the  rays  which  enter  between  c  li 
and  i  d  come  together  at  intermediate  points 
between  g  I  and  those  of  the  central  portion 
between  //  and  i  will  fall  beyond  /.  The  amount 
of  spherical  aberration  depends  upon  the  shape  of 
the  lens  and  in  different  lenses  of  the  same  focus, 
is  greatest  in  the  double  convex  form,  less  in  the 
plane  convex  and  least  in  the  so-called  crossed 
lens,  in  which  the  two  surfaces  are  of  different 
radii  and  in  the  proportion  of  1  to  6,  on  condition 
however,  that  the  surface  of  short  radius  is  directed 
toward  the  object.  This  form  of  lens  however,  is 
seldom  used,  as  it  shows  the  greatest  amount  of 
aberration  if  used  in  the  reversed  position.  The 


24 

most  common  form  is  the  double  convex  and  when 
considerable  magnifying-  power  is  desired,  the 
defects  of  spherical  aberration  are  partially  over- 
come by  the  interposition  of  an  opaque  plate  with 
round  opening  of  such  size  that  it  will  shut  out  the 
marginal  rays.  This  is  called  a  diaphragm  and 
when  used  the  lens  is  said  to  be  stopped  down. 

Chromatic  Aberration.  —  In  magnifying  an 
object  with  a  single  lens  it  will  be  noticed  that  it 
has  not  only  the  defect  of  spherical  aberration,  but 
that  the  object  is  fringed  with  colors,  predomi- 
nantly violet  and  red,  or  if  objects  are  viewed 
through  a  pri^m,  we  have  not  only  an  apparent 
change  of  position,  but  a  decided  appearance  of 
so-calied  rainbow  colors.  This  appearance  is 
called  chromatic  aberration  and  is  a  result  of 
refraction.  It  is  caused  by  the  dispersion,  or 
dispersive  quality,  the  separation  of  light  into  its 
primary  colors,  violet,  indigo,  blue,  green,  yellovv, 
orange  and  red  in  the  order  given.  As  the  light  is 
refracted  by  a  lens  or  prism  on  its  emergence,  the 
violet  end  being  more  refrangible  will  be  princi- 
pally affected  and  be  brought  to  a  focus  within, 
and  the  red  to  a  focus  beyond  the  principal  focus. 
This  band  of  colors  is  called  the  spectrum  and  is 
nicely  illustrated  in  the  diamond.  This  has  a  very 
high  degree  of  refractiveness  and  is  polished  or 
cut  to  make  a  many-sided  prism  in  which  each 
face  or  facet  creates  refraction  with  its  consequent 


25 


dispersion  and  play  of  colors.  To  avoid  to  the 
greatest  possible  extent  the  spherical  and  chro- 
matic aberration,  has  been  for  many  years  the 
study  of  opticians. 


Fig.  10. 

If  in  Fig.  10  the  beam  of  light  a  b  is  followed 
it  will  be  found  to  separate  at  the  first  surface  of 
the  first  prism  and  still  more  at  its  emergence  from 
the  second  surface.  The  violet  ray  undergoing 
the  greatest  amount  of  refraction,  takes  the  direc- 
tion of  v  v ',  and  the  red  ray  being  refracted  to 
a  lesser  extent,  follows  the  course  of  r  r .  To 
neutralize  or  correct  this  dispersion  a  second 
prism,  identical  with  the  first,  but  in11  a  reversed 
position,  may  be  placed  to  intercept  these  rays,  so 
that  they  will  be  recombined,  as  shown  in  r  c  and 
v  c  and  finally  emerge  as  white  light.  Now  while 
the  dispersion  can  be  corrected  in  this  manner,  it 
will  be  noticed  that  the  emergent  ray  c  d  from 
the  second  prism  takes  the  same  direction  as  the 
incident  ray  a  b  and  that  this  course  corresponds 


26 

to  the  diverging  rays  of  a  concave  lens.  It  will  be 
remembered  however,  that  in  order  to  have  image 
forming  rays  they  must  converge. 

As  dispersion  depends  upon  two  conditions,  the 
angle  of  the  prism  or  convexity  of  the  lens  and 
dispersive  power  of  the  glass,  it  will  be  seen  that 
in  two  prisms  of  the  same  angle  but  made  of  glass 
of  different  dispersive  power,  the  separation  of  the 
red  and  violet  rays  will  be  greatest  in  that  having 
the  greatest  dispersive  power.  Or,  in  two  prisms  of 
the  same  density  but  one  having  one  half  the 
angle  of  the  other,  the  dispersion  will  be  one  half 
as  great  in  the  prism  of  smaller  angle.  Again,  in 
the  prism  of  twice  the  angle  but  one  half  the  dis- 
persive power  of  another  the  dispersion  will  be  the 
same.  We  also  know  that  in  a  prism  having  a 
greater  angle  the  refraction  will  be  greater  than 
in  one  of  lesser  angle  and  as  a  refraction  must  be 
maintained  which  will  create  convergence  while  at 
the  same  time  neutralizing  the  dispersion,  a  solu- 
tion will  offer  itself  in  using  in  conjunction  with  a 
prism  of  ascertain  angle  and  dispersion,  another 
of  lesser  angle  and  greater  dispersion.  If  the 
same  principle  be  carried  out  in  lenses,  a  convex 
lens  of  low  dispersion  would  be  combined  with  a 
concave  lens  of  high  dispersive  power.  Such  a 
lens  is  shown  in  Fig.  11,  and  is  called  an 
achromatic  or  corrected  lens.  If  the  chromatic  and 
spherical  aberrations  are  both  corrected  it  is  called 


aplanatic.     The  convex  lens  is  made  of  crown  glass 
and  the  concave  of  flint  glass. 


Fig.  11. 

The  corrected  lens  shown  in  Fig.  11  is  an 
achromatic  lens  of  the  simplest  form  and  while  not 
absolutely  corrected  is  generally  used  when  the 
demands  on  it  are  not  too  great.  Better  correction 
is  obtained  when  two  or  more  of  these  lenses  are 
used  in  combination  and  they  are  thus  used  in 
some  of  the  lenses  of  the  compound  microscope. 
The  variety  of  forms,  due  to  the  variety  of  glass 
from  which  combinations  may  be  made,  is  almost 
infinite. 


SIMPLE   MICROSCOPES. 


Simple  microscopes  are  usually  termed  magni- 
fiers and  whether  consisting-  of  one  or  several 
lenses  in  close  contact,  always  remain  simple. 
They  are  made  to  be  held  in  the  hand  or  at'e  fixed 


Fig.  12.  Fig.  13. 

on  a  stand  or  mounting,  with  provisions  for  adjust- 
ing for  focus,  thus  to  give  steadiness  and  leave  the 
hands  free  for  dissecting  or  moving  the  object 


29 

during  observation.  They  are  made  in  a  large 
variety  of  forms,  the  difference  appearing  in  their 
optical  as  well  as  mechanical  construction.  The 
most  common  are  those  with  one  or  several  double 
convex  lenses  and  mounted  in  hard  rubber  or  vul- 
canite, as  shown  in  Fig.  12  and  Fig.  13.  Those 
containing  several  lenses  are  preferable,  since  they 
offer  a  variety  of  magnifying  powers  and  in  com- 
bination give  the  greatest  magnification  admissible 
with  single  lenses. 

Coddington  Lens. — A  lens  of  greater  efficiency 
than  the  ordinary  magnifier  is  the  so-called  Cod- 
dington  lens,  Fig.  14.  While  this  is  also  a  single 


O 


Fig.  14. 


double  convex  lens  it  will  be  noticed  that  it  has  con- 
siderable thickness,  being  really  the  central  portion 
of  a  sphere  and  provided  with  a  circular  incision 
at  the  middle,  which  is  blackened  and  thus  acts  as 
a  diaphragm,  shutting  out  the  marginal  rays  and 
correcting  the  spherical  aberration,  at  the  same 
time  however,  limiting  the  size  of  field. 


30 

Aplanatic  Triplet. — The  best  type  of  magnifier 
is  the  Aplanatic  Triplet,  Fig.  15,  being  composed 
of  three  lenses  cemented  together  in  such  a  man- 
ner that  they  are  virtually  one.  For  many  years 


Fig.  15, 

this  form  was,  on  account  of  cost,  used  only  to  a 
limited  extent,  but  an  increased  demand  has 
brought  about  a  greatly  reduced  price.  While  it 
corrects  both  aberrations  it  gives  a  large  and  flat 
field  and  can  be  commended  to  those  wishing  a 
good  magnifier. 

Hastings  Triplet. — This  form  of  lens  has  been 
recently  computed  by  Prof.  C.  S.  Hastings  of  Yale 
University  and  is  a  modification  of  the  Aplanatic 
Triplet,  giving  not  only  the  highest  spherical  and 
chromatic  corrections,  but  a  considerably  flatter 
and  larger  angular  field  and  longer  working  dis- 
tance; that  is,  the  distance  between  lens  and 
object  is  greater  and  is  an  important  feature  in 
higher  powers,  as  it  admits  of  better  illumination. 

In   all   of   the   better   types   of   magnifiers   the 


vulcanite  mountings  are  discarded  for  those  made 
of  metal,  especially  German  silver,  although  many 
are  used  of  pure  silver  arid  some  even  of  gold. 

Reading  Glass.  —  When  magnifying  lenses 
reach  a  diameter  of  2  inches  or  more  they  are 
usually  termed  Reading  Glasses  and  are  then  pro- 
vided with  a  handle.  They  are  used  to  enlarge 
small  printed  matter,  or  to  determine  detail  in 
engravings  and  photographs. 

Bruecke  Lens. — This  is  a  combination  of  lenses 
named  after  the  inventor  which,  while  at  one  time 
a  popular  construction  went  almost  into  disuse, 
but  in  recent  years  has  been  found  to  have  valu- 
able properties  and  is  now  very  generally  used.  A 
combination  of  achromatic  lenses  is  superposed  by 
an  achromatic  concave  lens  and  gives  a  very  satis- 
factory image.  Much  higher  power  can  be  ob- 
tained than  would  be  possible  in  any  other  form  of 
simple  microscope.  While  it  has  the  disadvantage 
of  a  small  field,  this  is  more  than  compensated  for 
by  a  much  larger  focal  distance  than  would  be  the 
case  in  the  ordinary  forms  of  simple  or  compound 
microscope,  thus  making  dissection  possible  during 
observation  and  a  further  advantage  over  the 
compound  microscope,  in  that  the  image  is  erect 
as  in  the  simple  form  and  not  reversed  as  in  the 
compound.  A  change  of  powers  is  made  conven- 
ient either  by  changing  the  lenses  or  varying  the 
position  of  the  eye  lens. 


32 

Stands  and  Holders. — There  is  a  great  variety 
of  mechanical  contrivances  for  holding  magnifiers, 
to  give  steadiness  as  well  as  to  leave  the  hands 
free  for  moving  and  working  on  the  object  and 
adjusting  for  focus.  When  provision  is  made  to 
hold  and  adjust  the  lens  the  apparatus  may  be 
called  a  lens  holder  or  stand,  but  when  a  platform 
called  a  stage  is  added,  upon  which  the  object  can 
be  placed,  and  a  mirror  for  illuminating  the  object 
properly,  it  is  called  a  dissecting  microscope. 

There  are  two  forms  of  magnifiers  which  contain 
within  themselves  some  properties  of  stands,  one  a 
Tripod  Magnifier,  Fig.  16,  rests  upon  three  legs 


Fig.  16. 

and  has  a  screw  for  adjustment  of  focus.  Its 
optical  parts  are  two  convex  lenses,  usually  having 
a  power  of  10  diameters,  and  which  are  separated 


33 

by  a  diaphragm,  giving  a  large,  fairly  flat  field. 
It  is  especially  used  in  primary  botanical  work. 

Another   is   the   Linen    Tester   (Fig.  17).     It   is 
made  in  various  sizes,  but  the  ordinary  form  has  a 


Fig.  17. 

lens  of  1  inch  focus.  It  is  arranged  to  fold  into 
small  compass  for  convenience  in  carrying  and 
when  opened  for  use,  is  placed  over  the  object  so 
that  this  comes  into  the  square  opening  in  the 
base.  Its  principal  purpose  is  however,  as  its 
name  indicates,  its  use  in  the  textile  industries  for 
counting  the  number  of  threads  which  appear  in 
the  standard  opening  of  \  inch. 

The  most  simple  form  of  holder  is  a  base  to 
which  is  fixed  a  stem  or  rod  upon  which  the 
magnifier  may  be  adjusted.  Fig.  18  shows  one 
with  a  white  opal  glass  plate  for  placing  the  object 
upon.  In  this  or  other  similar  forms  the  advantage 
of  holding  the  lens  is  counterbalanced  by  disad- 


35 

vantages,  such  as  the  necessity  of  holding  the  base 
when  adjusting  the  magnifier  and  the  danger  to 
the  eye  from  the  projecting  stem. 

The  Excelsior  Dissecting  Microscope  (Fig.  19) 
can  be  recommended  when  portability  is  a 
desideratum  and  for  school  use  in  primary  work, 
the  dissecting  microscope  designed  by  Prof.  C.  R. 
Barnes.  This  meets  many  conditions  of  a  dissect- 
ing microscope  in  a  simple  manner. 

There  is  a  variety  of  more  complex  forms, 
giving  a  number  of  advantages,  such  as  stability  ? 
convenience  of  varied  illumination,  delicate  means 
for  and  long  range  of  adjustment,  jointed  arms 
for  moving  the  lens  conveniently  over  a  large 
field,  etc.,  to  the  various  descriptions  of  which  the 
reader  is  referred  to  catalogues. 

How  to  use  Magnifiers  and  Dissecting 
Microscopes. — It  is  generally  admitted  that  the 
intelligent  use  of  a  magnifier  is  a  great  aid  in 
microscopical  studies  and  while  its  use  is  a  simple 
matter,  some  words  of  advice  may  be  of  aid  in 
obtaining  better  results,  or  lead  to  doing  work 
with  more  comfort.  In  all  work,  whether  with 
simple  or  compound  microscopes,  it  is  a  good  plan 
to  start  out  with  the  principle  not  to  use  a  greater 
magnifying  power  than  is  necessary  to  accomplish 
the  results  in  view. 

It  should  be  made  a  habit  at  the  outset  to  keep 
both  eyes  open. 


37 

Keep  the  eye  comfortably  near  to  the  upper 
surface  of  the  lens,  as  the  angular  view  is  in- 
creased and  there  is  the  least  spherical  aberration 
and  the  focal  distance  is  the  greatest.  This  can 
be  easily  tested  by  gradually  increasing  the  dis- 
tance between  the  eye  and  lens,  when  it  will  be 
found  that  the  lens  must  be  brought  nearer  to  the 
object.  In  single  lenses  the  spherical  and  chro- 
matic aberrations  become  more  pronounced  and 
the  field  smaller. 

As  magnifiers  which  are  used  on  opaque  objects 
—  those  which  are  not  transparent  and  which  are 
illuminated  by  reflected  light  not  transmitted 
through  the  object  —  a  position  should  be  chosen 
toward  a  window  or  flame,  which  will  allow  the 
greatest  amount  of  light  to  reach  the  object.  If  a 
hat  is  worn,  place  this  back  on  the  head  so  that 
the  rim  or  shield  will  not  cut  off  the  light. 

In  holding  the  object  in  one  hand,  take  the 
magnifier  between  the  thumb  and  forefinger  of 
the  other  and  place  the  fingers  of  hand  holding 
the  lens  in  such  a  manner  that  they  shall  rest 
upon  the  other  hand ;  this  will  insure  steadiness 
between  the  lens  and  object  and  will  add  consider- 
ably to  the  comfort  of  working. 

While  it  seldom  occurs  that  magnifiers  are  made 
with  other  than  double  convex  surfaces,  single 
achromatic  lenses  are  sometimes  used  which  are 
plane  convex.  In  these  the  plane  side  should 


always  be  toward  the  eye.  In  the  case  of  two 
plane  convex  lenses,  they  should  be  used  with 
their  convex  surfaces  toward  one  another. 

In  magnifiers  containing  several  lenses,  when 
these  are  used  together,  the  one  of  highest  power 
should  be  nearest  the  object.  When  reversed  the 
angular  field  is  greater,  but  presents  considerable 
spherical  and  chromatic  aberration,  which  it  is 
advantageous  to  limit. 

In  simple  dissecting  microscopes  like  the 
Barnes,  in  which  the  mirror  is  in  a  fixed  position, 
the  microscope  should  be  set  squarely  before  the 
source  of  light.  Diffused  light,  such  as  daylight, 
is  always  preferable  to  any  artificial  illumination. 

It  should  always  be  sought  to  modify  the  light 
as  much  as  possible  and  still  have  enough  to  see 
easily  as  the  eyes  are  much  less  fatigued. 

While  in  some  classes  of  work  it  is  perhaps  un- 
necessary to  have  the  very  best  magnifiers,  such 
as  the  Aplantic  or  Hastings  Triplets,  the  latter 
can  always  be  recommended  when  the  means  will 
permit  on  account  of  the  higher  results  and 
greater  degree  of  satisfaction  and  comfort  derived 
from  them. 

Magnifying  Power.  —  Unless  a  microscope, 
whether  simple  or  compound,  is  known  to  come 
from  the  hands  of  a  reliable  firm,  any  claim  as  to 
magnifying  power  should  be  accepted  with  re- 
serve. In  former  years,  when  the  country  was 


39 

overrun  with  cheap  foreign  productions,  the  most 
fanciful  claims  were  made  in  this  direction.  Avoid 
strolling  or  street  venders  who,  as  a  rule,  not  only 
make  the  most  ridiculous  claims  as  to  magnifying 
power,  but  charge  much  higher  prices  than  the 
same  articles  can  be  bought  for  from  reliable 
opticians  and  generally  offer  articles  of  worthless 
or  at  best  doubtful  value. 

Some  precaution  should  also  be  used  in  refer- 
ence to  quality  in  purchasing  a  magnifier.  As 
competition  causes  a  downward  tendency  in  prices, 
it  unfortunately  often  involves  a  deterioration  in 
quality.  The  ordinary  forms  are  mounted  in 
vulcanite  ;  black  horn  is  often  palmed  off  as  such, 
but  is  a  poor  substitute  as  it  warps  and  cracks. 
The  surfaces  of  lenses,  instead  of  being  perfectly 
polished  are  often  scratched  and  unfinished  show- 
ing small  pit-holes  or  undulating  surfaces.  This 
is  common  among  cheap  Coddington  lenses  and 
naturally  prevents  obtaining  a  distinct  image. 

It  is  evident  that  a  lens  magnifies  an  object 
equally  in  all  directions  ;  this  is  said  to  be  in  areas, 
and  is  the  square  of  the  linear,  so  that  if  an  object 
is  magnified  four  times  in  the  linear,  it  is  sixteen 
times  in  area.  The  commonly  accepted  term  to 
express  magnifying  power  of  simple,  as  well  as 
compound  microscopes,  is  in  diameters  (linear). 
A  single  lens  of  1  inch  focus  magnifies  about  ten 
diameters;  one  of  -2  inch  focus,  about  five  diameters  ; 


OP  THE 

TJNIVERSmr 


40 

one  of  |  inch  focus,  twenty  diameters  and  so  on. 
In  a  lens  of  high  magnifying  power,  the  focus  is 
ordinarily  about  twice  the  diameter,  so  that  if 
a  lens  is  \  inch  diameter  its  focus  is  about  1  inch. 
To  Determine  Magnifying  Power. —  While 
the  determination  of  focus  in  single  lenses  gives 
approximate  magnifying  power,  it  cannot  be  done 
in  some  of  the  forms  which  have  been  described. 
The  following  method,  if  carefully  followed,  will 
give  very  accurate  results  and  is  withall  simple 
and  interesting.  Upon  the  upper  or  farther  edge 
of  a  sheet  of  white  paper,  which  rests  upon  a  table 
with  the  light  at  the  right  or  left  hand,  place  a 
series  of  books  of  such  height  that  when  the  mag- 
nifier is  placed  upon  the  inner  edge  or  that  nearest 
the  body,  of  the  upper  book  with  the  lens  project- 
ing over  it,  the  distance  to  the  upper  surface  will 
be  exactly  10  inches  and  weigh  down  the  mount- 
ing with  an  additional  book  or  weight.  Now 
place  a  pocket  rule  between  the  leaves  of  upper 
book  so  that  when  the  edge  is  close  to  the  magni- 
fier the  divisions  on  the  rule  will  be  exactly  in 
focus,  place  the  rule  so  that  it  shall  not  project 
much  over  the  lens.  It  is  immaterial  what  the 
divisions  on  the  rule  are,  whether  inches  or  milli- 
meters, so  long  as  they  are  reasonably  fine.  View 
the  divisions  with  the  right  eye  and  open  the  left 
eye,  when  it  will  be  found  that  the  divisions  are 
apparently  projected  upon  the  paper.  Take  a 


41 

pencil  and  outline  upon  the  paper  one  of  the 
spaces.  By  dividing  the  actual  number  of  divi- 
sions on  the  rule  into  this  enlarged  space  the  exact 
magnifying  power  will  be  determined.  Thus,  if  it 
is  found  that  five  spaces  are  contained  in  the  one 
space  on  the  paper,  the  magnifying  power  is  five  and 
the  focus  of  the  lens  2  inches,  or  if  ten  spaces,  it  is 
ten  with  a  focus  of  1  inch.  One  or  several  lenses 
in  conjunction  may  be  examined  in  this  way. 
Some  difficulty  may  or  probably  will  be  experi- 
enced in  seeing  the  divisions  on  the  rule  and  on 
the  paper  at  the  same  time,  but  this  will  be  over- 
come with  a  little  practice.  Indeed,  it  is  well  to 
point  out  at  this  stage  that  both  eyes  should  be 
kept  open  in  viewing  objects  through  simple  as 
well  as  compound  microscopes,  as  continued  work 
can  be  done  with  infinitely  more  comfort  and 
while  at  first  some  difficulty  may  be  experienced, 
it  will  be  found  that  after  a  little  earnest  effort, 
both  eyes  unconsciously  remain  open  and  the 
prominence  with  which  objects  have  appeared  to 
the  unoccupied  eye  is  less,  as  the  mind  becomes 
intent  upon  the  object  it  is  -viewing.. 


THE  COMPOUND  MICROSCOPE. 


As  has  been  stated  a  magnified  image  is  observed 
in  the  Compound  Microscope.  Any  two  lenses, 
one  of  short,  the  other  of  long  focus,  placed  suffi- 
ciently far  apart,  will  attain  this  object  and  this 
was  for  years  the  method  of  its  construction. 

In  any  microscope,  whether  simple  or  com- 
pound, the  difficulty  of  holding  it  or  the  object 
steady  during  observation,  increases  with  the 
increase  in  magnifying  power  and  in  the  com- 
pound form  with  only  a  moderately  high  power, 
it  is  utterly  impossible  to  retain  sufficient  steadi- 
ness to  make  any  reliable  observation.  Mechanical 
contrivances  therefore  became  a  necessity  and 
were  applied  in  the  very  earliest  constructions  of 
the  microscope.  Even  when  such  a  luxury  as  an 
achromatic  lens  was  unknown  they  were  all  made 
to  fulfill  the  following  conditions  : 

A  platform  for  holding  the  object. 

A  means  of  adjustment  for  properly  focusing  on 
the  object. 

Provisions  for  suitably  illuminating  the  object. 

From  what  may  be  called  a  crude  attainment  of 
of  these  three  purposes,  the  construction  gradually 
became  more  complex.  Many  additions  have 


43 

been  made  which  have  proven  useful  and  have 
remained,  while  others  have  been  discarded.  As 
the  first  microscope  was  constructed  kf  1590  itfhas 
required  nearly  three  centuries  to  bring  the  in- 
strument up  to  its  Ipresent  general !  form  and  it  is 
interesting  to  note  that  ;  many  improvements 
which  have  been  introduced  within  the  last  forty 
or  fifty  years  have  been  used  and  lost  sight  of 
within  this  time. 

While  certain  parts  are  necessary  to  make  up  a 
modern  instrument,  no  one  design  of  construction 
is  followed.  The  forms  are  innumerable,  each 
maker  following  his  own  inclination  in  variety, 
design,  number  of  parts  and  material.  For  the 
latter  brass  predominates,  although  bronze  and 
iron  are  used  to  a  considerable  extent.  The  first 
two  metals  are  usually  highly  finished  and  as  they 
easily  tarnish,  are  protected  by  lacquer,  which  is 
not  only  serviceable  in  this  direction  when  well 
done,  but  offers  a  means  of  ornamentation.  Iron 
is  covered  with  a  heavy  coating  of  japan  and  be- 
ing dark  is  on  this  account  often  recommended  by 
instructors  as  being  agreeable  for 'the  eyes.  The 
entire  apparatus,  including  the  optical  parts,  is 
called  a  microscope,  whereas,  without  them,  it  is 
termed  a  stand.  Some  people  call  it  a  "machine," 
but  we  earnestly  protest  against  this  harsh  term 
as  applied  to  an  instrument  of  such  precision  as 
the  microscope  is. 


M 


Fig.  20. 


45 

Description  of  Parts.  —  As  it  is  necessary  for 
the  student  to  become  conversant  with  the  terms 
of  the  various  parts  and  to  understand  their  use, 
we  give  an  illustration  (Fig.  20)  with  letters 
and  append  a  list  giving  the  names  and  recom- 
mend that  they  be  impressed  upon  the  memory, 
as  they  are  the  basis  of  microscopical  language. 

A.  Base. — This  is  the  foundation  of  the  instru- 
ment.     It   usually    rests    upon    three   points    (or 
should  do  so)  and  is  of  such  a  weight  that  it  keeps 
the  instrument  firm  when  it  is  in  an  upright  or 
inclined  position.      The  two  principal  forms  are 
the  tripod  and  horseshoe.      The  revolving  plate, 
when  this   is   provided,   by   means  of  which  the 
upper  portion  of  the  instrument  is  revolved,  with- 
out changing  the  position  of  the  base,  is  considered 
a  part  of  it. 

B.  Pillar.1 — It  is  the  vertical  column  which  is 
fastened  to  the  base  and  carries  upon  its  upper 
end  the  joint  or  axis  which  is  provided  for  inclin- 
ing the  instrument.     It  generally  consists  of  one 
piece  either  round  or  square  but  is  often,  in  larger 
instruments,  made  in  two  columns. 

C.  Arm. —  This  supports  all  the  upper  working 
parts  of  the  instrument  and  carries  the  provisions 
for  adjusting  for  focus. 

D.  Body. —  This  is  the  tube  portion  to  which 
the  optical  parts  are  attached. 


46 

E.  Nose-Piece. —  This  is  an  extra  piece  which 
is  attached  to  the  lower  part  of  the  tube. 

Society  Screw. —  This  is  a  standard  screw 
which  is  cut  into  the  nose-piece  and  is  called  so 
from  the  fact  that  it  was  first  established  by  the 
Royal  Microscopical  Society  of  London.  It  is 
also  called  the  universal  screw  and  is  in  general 
use  in  this  country  and  England ;  it  has  lately 
been  adopted  by  some  firms  on  the  continent  of 
Europe. 

F.  Objective. —  This  is  screwed  into  the  nose- 
piece   and  is  called   so  because  it  is  nearest  the 
object.     It  is  the  most  important  of  the  two  opti- 
cal parts  (of  the  microscope  proper)  and  upon  its 
perfection    the    distinctness    of    the    image    and 
therefore   the    value    of    the    instrument    almost 
entirely  depends. 

G.  Eyepiece  or  Ocular. — It  is  called  so  because 
it  is  nearest  the  eye  and  is  the  remaining  optical 
part.     It  magnifies  the  image  given  by  the  objec- 
tive.    This   and    objective  .will  be    fully    treated 
later  on. 

H.  Draw-Tube. — This  is  that  portion  of  the 
body  which  moves  in  the  outer  sheath  and  which 
receives  the  eyepiece.  It  is  provided  for  the  pur- 
pose of  attaining  different  lengths  and  variations 
in  magnifying  power. 


47 

I.  Collar. — This  is  a  ring  which  is  attached  to 
the  draw-tube  and  is  usually  provided  with  a  milled 
edge. 

J.  Coarse  Adjustment. — This  is  a  provision 
for  moving  the  body  quickly  back  and  forth  for 
adjusting  the  focus  approximately.  It  is  done  by 
a  sliding  rack  and  stationary  pinion  (not  shown  in 
cut)  or  a  sliding  body  in  an  outer  sheath. 

K.  Milled-Heads. — These  are  attached  to  the 
shank  of  the  pinion,  which  is  revolved  by  means 
of  them  and  are  usually  large  to  give  sensitiveness 
to  the  movement.  They  should  be  placed  wide 
apart  so  that  the  fingers  may  be  entirely  free  from 
the  body. 

L.  Fine  Adjustment. — This  is  slow  moving 
and  serves  to  get  an  exact  focus.  It  is  attained  by 
a  fine  screw,  provided  with  a  milled  head  and  acts 
upon  the  body,  either  directly  or  by  levers.  This 
as  well  as  the  coarse  adjustment  should  be 
extremely  sensitive  and  should  not  have  the  least 
side  or  lateral  motion.  The  fact  that  either  of 
them  have  it,  is  evidence  of  poor  workmanship. 

M.  Stage. — This  is  the  portion  on  which  the 
object  is  placed  for  examination  and  is  usually 
attached  to  the  arm,  although  the  arm  is  sometimes 
attached  to  the  stage. 


48 

N.  Clips. — These  are  two  springs  which  are 
attached  to  the  upper  surface  of  the  stage  and 
serve  to  hold  down  the  object. 

Centering  Screws. — These  are  provided  for 
moving  the  stage  in  different  directions  to  bring 
the  center  of  its  revolving  motion  in  the  center  of 
the  field. 

O.  Mirror. — This  is  used  for  reflecting  and  con- 
densing light  upon  the  object.  As  a  rule  two 
mirrors  are  used,  one  plane  and  the  other  concave. 
The  first  gives  a  comparatively  weak  light,  while 
the  second  concentrates  it  and  gives  it  more 
intensity. 

P.  Mirror  Bar. — This  carries  the  mirror  and 
swings  in  a  circle  around  the  object  in  order  to 
illuminate  it  from  various  directions. 

Q.  Substage. — This  is  a  ring  or  attachment 
below  the  stage  to  receive  various  accessories 
which  may  be  required.  It  is  sometimes  fixed  to 
the  stage  but  in  the  best  instruments  it  is  separated 
from  it  and  is  provided  with  an  adjustment  to 
vary  its  distance  from  the  object. 

Substage  Bar. — This  receives  the  substage 
and  permits  its  adjustment.  In  modern  American 
instruments  this,  as  well  as  the  mirror-bar,  is  on 
an  axis  in  the  plane  of  the  stage,  so  that  whatever 
position  they  may  be  in,  relative  to  the  object,  the 


49 

distance  from  this  to  the  substage  or  mirror  does 
not  vary,  except  when  made  to  do  so. 

S.  Diaphragm. — This  is  a  provision  for  stopping- 
down  or  regulating  the  amount  of  light  which 
illuminates  the  object. 

Optical  Axis. — This  is  an  imaginary  line  which 
passes  from  the  center  of  the  eyepiece  through 
the  centers  of  the  body,  objective,  stage  and  sub- 
stage  to  the  mirror.  Whatever  lies  in  it  is  said  to 
be  centered. 

Object. — That  which  is  examined. 

Slide  or  Slip. — This  is  a  thin  plate  of  glass 
upon  which  the  object  is  placed  or  mounted,  the 
prevailing  standard  being  3  inches  long  by  1  inch 
wide. 

Cover  Glass. — This  is  an  extremely  thin  piece 
of  glass,  round  or  sqtiare,  which  is  placed  upon 
the  object,  either  for  flattening  or  preserving  it,  or 
both. 

Classification  of  Microscopes. — Until  recently 
microscopes  were  divided  into  two  classes,  the 
Jackson  and  the  Ross  models.  While  the  latter 
was  for  many  years  very  popular,  particularly 
with  the  English  makers,  it  has  been  almost 
entirely  superseded  by  the  Jackson  form  and  with 
good  reason.  In  the  former  the  means  of  adjust- 
ing were  provided,  as  near  as  consistent  with  the 
construction,  to  the  body  or  tubes,  whereas  in  the 


r>o 

Ross  they  are  placed  at  the  back  or  more  distant 
point  in  the  instrument,  thus  increasing-  by  means 
of  the  connecting-  arm  the  faults  which  might  exist 
in  the  adjustment. 

A  certain  form  of  instrument  which  at  the 
present  is  very  popular  and  called  the  Continental 
pattern,  from  the  fact  that  it  was  made  originally 
by  the  manufacturers  on  the  Continent  of  Europe, 
is  a  combination  of  both  the  Jackson  and  Ross 
models.  Whereas  the  coarse  adjustment,  when 
consisting  of  a  rack  and  pinion,  is  placed  closely  to 
the  tubes,  the  fine  adjustment  is  placed  in  the  arm. 

There  is  another  direction,  however,  in  which 
microscopes  are  divided  into  two  classes,  which  is 
of  far  more  importance,  and  affects  their  utility  in 
a  much  higher  degree.  The  writer  does  not  know 
that  instruments  have  been  so  classified  by  others, 
but  knows  that  they  can  be  with  perfect  propriety. 
The  distance  between  the  eyepiece  and  the  objec- 
tive is  one  of  vital  importance  in  the  optical  results 
and  as  there  are  several  lengths  of  tube,  the  optical 
qualities  of  an  objective  are  injuriously  affected 
if  it  is  used  with  a  different  length  than  that  for 
which  it  was  originally  intended.  It  is  therefore 
proposed  to  classify  microscopes  according  to  their 
tube  length  as  long  tube  ancLs*/w/  /W;r  instruments. 

Tube  Length. — In  the  Continental  form  (Fig. 
-20),  a  short  tube  from  160.0  to  170.0  mm.  (6.8  to  6.7 
inches)  is  used,  whereas  in  the  English  form,  this 


51 


Pts.  included 
in  Tube-length. 
See  Diagram. 
a 

Tube-length 
in 
Millimeters. 

203 

rz 

/^ 

-X\ 

-H£              rr» 

f  Grunow, 

E.  Leitz, 

170 

- 

~c             Nachet  et  -Fils, 

146  or  200 

«-*/•{  Powell  and  Lealand, 

254 

C.  Reichert, 
Spencer  Lens  Co.  , 

160  to  180 
235  or  160 

^ 

—  -^ 

X 

1  W.Wales,      - 

254 

Bausch  &  Lomb  Opt.  Co., 

216  or  160 

Bezu,  Hausser  et  Cie., 

220 

f    ,    Klonne  und  Muller, 
o-a  + 

W.  &  H.  Seibert, 

160  -180  or  254 
190 

Swift  &  Son, 
[C.  Zeiss, 

165  to  228£ 
160  or  250 

tf 

$ 

a      \  Gundlach  Optical  Co., 
(  R.  Winkel, 

254 
220 

I* 

*H  -^  o</    Ross  &  Co., 

254 

•N 

^—  — 
^-_ 

'  -v 

—  -* 

x-1 

„  e     c-e    R.  &  J.  Beck,      - 
c-f    J.  Green, 
Hartnack, 

254 
254 

160-180 

v  — 

^ 

_                Verick, 
«s 
Watson  &  Sons, 

160-200 
160-250 

Fig.  21. 


52 

being  largely  followed  in  America,  the  length  is 
from  8J  to  10  inches  (216.0  to  250.0  mm.)  The 
short  tube  of  the  European  makers  offers  no 
optical  advantages,  but  is  mainly  used  to  contract 
the  height  of  the  instrument  to  as  great  an  extent 
as  possible,  as  this  is  the  vital  point  throughout  its 
construction. 

Until  recently  this  subject  was  given  little  atten- 
.  tion,  each  maker  following  a  standard  which  he 
had  adopted  for  himself.  The  pernicious  influence 
of  this  diversity  was  not  appreciated  by  the  public, 
as  it  was  not  acquainted  with  the  products  of  dif- 
ferent makers,  until  Prof.  S.  H.  Gage  made  it  the 
subject  of  a  paper  before  the  American  Society  of 
Microscopists  and  as  a  result  of  his  inquiries, 
tabulated  the  standards  as  followed  by  the  different 
makers,  which  is  published  herewith. 

As  a  result  of  his  reports  a  committee  was 
appointed  to  consider  this  subject,  as  well  as  that 
of  eyepiece,  objective  and  thickness  of  cover  glass, 
to  which  we  will  recur  farther  on  and  reported  in 
favor  of  the  adoption  of  two  standards  for  tube 
length. 

Short  standard  160.0  mm.  (6.3  inches)  ;  long 
standard  216  mm.  (8.5  inches)  ;  that  the  tube 
length  shall  be  considered  those  parts  between  the 
upper  end  of  the  tiibe  where  the  ocular  is  inserted 
and  the  lower  end  of  the  tube  where  the  objective 
is  inserted.  While  the  European  makers  have  paid 


53 

little  heed  to  this  standard,  we  believe  that  all  of 
the  American  firms  follow  it  and  if  it  has  accom- 
plished nothing  with  the  foreign  producers,  it  is  at 
any  rate  a  mean  between  the  standards  of  the 
leading  makers  and  with  the  publicity  which  the 
matter  has  received,  gives  the  user  an  opportunity 
to  use  his  intelligence  to  obtain  the  best  results. 
There  are  no  optical  advantages  in  the  one  or  the 
other.  The  short  length  is  almost  a  necessity 
however,  in  the  Continental  pattern  of  micro- 
scopes as  compactness  is  the  special  desideratum, 
but  while  this  subject  will  be  given  more  extended 
attention  and  optically  considered  farther  on,  it 
might  be  here  stated  that  when  an  objective, 
except  perhaps  in  the  very  low  powers,  is  con- 
structed to  be  used  with  a  certain  length  of  tube, 
it  should  be  used  with  this  length  only.  This 
statement  cannot  be  made  too  prominent  and  will 
bear  repetition. 

Stage. — This  being  the  plate  or  platform  on 
which  the  object  is  placed,  it  should  be  of  a 
strength  to  stand  the  weight  of  the  finger  under 
considerable  magnification.  This  depends  upon 
the  material  of  which  it  is  made  and  its  thickness, 
but  since  the  material  is  virtually  the  same  in  all 
instruments,  we  must  depend  upon  the  thickness 
for  rigidity.  Absolute  rigidity  is  practically  impos- 
sible when  considerable  force  is  exerted,  as  can 
easily  be  determined  in  the  best  instruments  and 


54 

it  is  a  mistake  to  condemn  an  instrument  for  this 
cause  as  is  sometimes  done.  If  the  object  will 
remain  in  focus  under  a  high  power  with  a  fair 
amount  of  exertion  above  that  which  is  required  in 
moving  the  object  about,  the  stability  may  be  con- 
sidered ample.  In  fact  in  this,  as  well  as  other 
directions,  the  writer  considers  it  unwise  for  a 
user  of  a  microscope  with,  in  most  cases,  a  neces- 
sarily limited  knowledge  of  its  construction,  to 
make  assertions,  as  is  often  done,  as  to  supposed 
defects  against  the  long  experience  of  the  micro- 
scope maker,  although  it  is  the  purpose  of  this 
book  to  aid  the  reader  in  judging  of  defects  when 
such  exist.  In  older  instruments  the  fault  often 
occurred  of  making  the  stage  unnecessarily  thick, 
which  interfered  with  the  proper  use  of  substage 
accessories  and  the  mistake  has  in  recent  years 
been  made  by  some  makers  of  having  it  too  thin, 
so  that  a  slide  could  not  be  moved  except  with 
the  utmost  care  without  depressing  it  and  thus  put- 
ting the  object  out  of  focus.  At  the  present  time, 
however,  the  writer  believes  that  in  instruments  of 
the  best  makers  the  thickness  is  ample,  although 
in  cheap  foreign  ones  he  knows  that  it  is  some- 
times not  the  case.  It  is  of  the  greatest  impor- 
tance however,  that  the  surface  of  the  stage 
should  be  square  with  the  tube  in  all  directions. 

The  lower   surface   is  generally  blackened  and 
the  upper  surface  brightly  polished  and  lacqiiered 


or  blackened.  From  the  constant  friction  of  mov- 
ing the  slide  around,  the  lacquer  or  artificial 
blacking  becomes  worn  and  in  time  the  stage 
assumes  a  shabby  appearance.  To  overcome  this, 
glass  has  been  used  with  more  or  less  success,  but 
in  recent  years  vulcanite  has  come  into  use  and 
has  proven  very  successful.  The  peculiar  gritty 
feeling  due  to  small  particles  of  dust  between  the 
stage  and  slide  is  not  so  noticeable  as  on  a  metal 
surface  and  it  is  not  affected  by  acids  or  alkalies 
and  will  therefore  retain  its  neat  appearance 
almost  indefinitely. 

Revolving  Stage. — While  in  the  largest  num- 
ber of  instruments  the  stage  is  fixed  and  either 
round  or  square,  there  are  others  which  are  revolv- 
ing, that  is,  may  be  revolved  around  the  optical 
axis.  These  are  absolutely  necessary  in  the 
examination  of  crystals  and  rock  sections  and  are 
then  arranged  with  graduations  around  the  edge — 
a  series  of  divisions  reading  to  degrees  or  fractions 
of  them — by  means  of  which  the  angles  of  the 
objects  are  measured.  As  a  slight  deviation  of 
the  center  of  revolving  motion  from  exact  coinci- 
dence with  the  optical  axis  will  cause  the  object  to 
swing  out  of  the  field,  centering  screws  are  pro- 
vided by  which  this  error  can  be  quickly  corrected. 
It  sometimes  occurs  as  the  stage  is  revolved,  that 
an  object  at  the  edge  of  the  field,  which  is  in  focus, 
gradually  becomes  indistinct,  showing  poorest  at 


56 

the  half  revolution  and  as  the  stage  is  brought 
around  to  the  first  point  again  comes  into  focus. 
This  may  be  due  to  poor  fitting  of  the  parts,  or  to 
the  fact  that  the  stage  is  not  square  in  all  direc- 
tions with  the  body ;  in  either  case  a  serious 
defect. 

Glass    Stage. — This    with     the    slide    carrier 
(Fig.  22)  is  a  device  for  moving  the  object  more 


Fig.  22. 

steadily  and  smoothly  than  can  be  done  directly 
on  the  stage.  It  is  made  detachable  and  consists 
of  a  glass  plate  in  a  metal  frame  upon  which  rests, 
on  four  points,  the  slide  carrier.  At  its  ends  the 
spring  arms  are  passed  around  the  glass  plate  and 
press  against  its  lower  surface,  thus  offering  the 
minimum  of  friction,  with  sufficient  resistence  to 
make  an  easy  movement. 

Mechanical  Stage. — This  is  a  very  important 
form  in  which  the  movements  are  mechanical  in 
two  directions  and  at  right  angles  to  one  another, 


57 

being  transmitted  through  %the  milled  heads  by  a 
rack  and  pinion  or  screw  movement.  If  pecuniary 
means  will  permit  it,  it  is  a  most  useful  addition 
and  by  it  work  can  be  done  systematically  and 
with  the  assurance  that  every  portion  of  the  field 


Fig.  23. 

has  been  covered  and  withall  with  a  degree  of 
comfort  which  must  be  experienced  to  be  appre- 
ciated. For  instance,  in  a  bacteriological  or 
urinary  specimen,  where  one  is  searching  the  field 


5s 

for  certain  appearances,  it  is  the  only  reliable 
means  of  covering"  each  portion  of  it.  This  stage 
is  especially  valuable  for  blood  counting-  and 
plankton  work.  While  not  many  years  ago  it  was 
spurned  as  a  toy  by  many  scientists,  it  is  now 
generally  accepted  as  an  invaluable  part  of  a 
microscope.  In  order  to  be  so  however,  it  must  be 
of  the  most  perfect  workmanship,  which  is  difficult 
to  attain  on  account  of  the  necessarily  small  parts 
of  which  it  is  composed  and  the  hard  usage  which 
it  must  bear.  The  movement  must  be  smooth  and 
easy  and  on  reversing  the  milled  heads,  must  not 
show  any  lost  motion  or  dead  point. 

There  is  a  variety  of  forms,  but  two  principal 
types,  one  of  which  is  built  onto  the  stage,  usually 
the  revolving  one,  the  other  attachable  (Fig.  23) 
which  may  be  removed  when  not  wanted,  leaving 
the  ordinary  stage  intact.  They  are  made  with 
graduations,  usually  divisions  in  millimeters,  by 
which  one  may  read  off  the  amount  of  space  which 
is  covered. 

In  using  the  mechanical  stage  it  "should  first  be 
determined  how  many  spaces,  or  how  much  of  one 
space  is  contained  within  the  limits  of  the  field  ; 
then  begin  at  one  edge  of  the  specimen  and  with 
lateral  movement  make  the  object  pass  across 
the  field.  Move  the  object  forward  with  the  other 
movement  the  amount  of  space  which  has  been 
previously  determined  and  by  a  return  motion 


of  the  lateral  movement,  bring-  it  across  again  and 
thus  through  the  entire  specimen,  or  until  the 
object  is  found  or  the  specimen  searched  over. 

Nose-Piece. — This  being  the  lower  end  of  the 
body,  to  which  the  objectives  are  attached,  it  is 
important  in  so  far  as  it  must  be  accurately  made. 
As  stated  it  has  the  society  screw.  Previous  to 
1857  each  maker  followed  a  standard  of  his  own 
and  this  to  a  great  extent  is  still  the  case  on  the 
Continent.  The  Royal  Microscopical  Society  of 
London  appreciating  the  inconvenience  of  this 
diversity,  recommended  a  standard  thread  of  36  to 
the  inch  with  an  external  diameter  of  0.8  inch, 
which  was  finally  adopted  in  England  and  this 
country.  The  Society  supplied  to  the  makers  a 
so-called  standard  hob  or  tap  with  which  to  gauge 
the  thread.  Unhappily,  however,  these  taps  are 
not  to  a  standard,  as  there  is  a  variation  in  those 
which  are  sent  out  by  the  Society,  so  that  while  the 
public  is  under  the  impression  that  there  are  fixed 
dimensions  there  is  a  diversity  in  the  products  of 
different  makers,  so  that  it  often  happens  that  the 
objectives  of  one  maker  will  not  enter  the  stand 
of  others.  The  writer  in  1884  read  a  paper  on  this 
subject  before  the  American  Society  of  Micro- 
scopists  and  as  a  result  a  committee  was  appointed 
to  bring  about  a  better  state  of  affairs.  It  failed, 
however,  in  obtaining  the  co-operation  of  the 
Royal  Microscopical  Society,  the  main  reason 


60 

being  the  expense  involved,  so  that  we  must  con- 
trive to  suffer  until  some  concerted  action  is  taken 
by  the  manufacturers  themselves,  which  we  trust 
will  not  be  too  far  distant. 

Double  Nose-Piece. — In  changing-  one  objec- 
tive for  another  to  obtain  another  power,  time  is 
consumed  and  it  is  often  inconvenient ;  besides, 
there  is  danger  of  dropping  the  objective  or  dis- 
turbing or  destroying  the  object.  To  avoid  this 
the  double  (Fig.  24)  the  triple  and  quadruple  nose- 
pieces  are  offered,  to  the  first  of  which  two,  to  the 


Fig.  24. 

next  three  and  to  the  quadruple  four  objectives 
may  be  attached  in  such  a  manner  that,  when  fixed 
to  the  noseTpiece  of  the  tube,  each  objective  may 
be  in  turn  brought  into  use  by  swinging  the  nose- 
piece  and  in  such  a  manner  that  each  is  centered 
and  will  be  in  focus,  if  not  exactly,  at  any  rate  very 
closely.  Of  all  the  conveniences  in  accessories 
these  are  the  most  useful  and  in  most  common 
use,  the  writer  knowing  from  his  experience  as 


manufacturer  that  80 
per  cent,  of  the  in- 
struments sold  for 
personal  use  are  sup- 
plied with  a  double 
nose-piece  when  two 
objectives  are  used, 
and  the  triple  when 
three  are  taken. 

Bodies  or  Tubes. 

These  are  divided 
into  two  classes,  mo- 
nocular, having  one 
body  with  which 
observation  is  made 
with  one  eye,  which 
may  contain  one  or 
two  draw-tubes  and 
binocular  for  observa- 
tion with  both  eyes, 
in  which  the  tubes 
are  fixed  together  at 
the  nose-piece  and 
gradually  separate 
until  they  reach  the 
pupillary  distance. 
The  first  would  be  a 
monocular  microscope, 
the  second  a  binocular 
microscope. 


Fig.  25. 


62 

While  the  methods  for  transmitting  the  rays 
from  the  objective  to  the  binocular  tube  vary,  the 
construction  in  most  common  use  is  that  introduced 
by  Mr.  Wenham. 

By  reference  to  .Fig.  -25  it  will  be  seen  that  the 
rays  from  one-half  the  objective  are  transmitted 
uninterruptedly  to  the  vertical  tube,  while  the 
prism  intercepts  the  rays  from  the  other  half  and 
by  reflection  induces  them  to  pass  into  the  oblique 
tube.  The  result  is  an  image  in  each  eyepiece 
thus  giving  stereoscopic  vision.  This  gives  a  per- 
ception of  depth,  a  peculiar  faculty  of  being  able 
to  look  into  an  object  and  conveys  to  the  mind  the 
impression  of  roundness  or  separation  of  the 
object  into  different  planes  which  it  is  impossible 
to  obtain  with  monocular  vision.  Its  use,  how- 
ever, is  limited  to  the  lower  power  objectives. 

Coarse  Adjustment. — In  providing  this  adjust- 
ment, two  methods  are  followed.  The  most  simple 
form  is  that  by  a  sliding  tube  in  which  the  tube 
which  carries  the  nose-piece  at  the  lower  end  and 
draw-tube  at  the  upper  end,  is  moved  up  and  down 
in  an  outer  sheath,  which  is  fastened  to  the  arm. 
The  milled  ring,  which  is  provided,  is  ^grasped  by 
thumb  and  fore  and  middle  fingers  and  pushed 
down  or  drawn  up  by  a  spiral  motion.  It  is  not  to 
be  commended  except  for  economical  reasons,  as 
it  lacks  firmness,  wears  out  quickly  from  the  con- 
siderable friction,  endangers  the  object  and  the 


63 

objective  from  the  liability  of  sudden  or  jerking' 
motions  and  does  not  well  permit  the  application 
of  the  double  nose-piece.  While  the  clamping- 
ring  is  provided  in  some  instruments,  which  will 
fasten  the  tube  in  a  fixed  position,  especially  to 
permit  the  use  of  double  nose-piece,  this  again  has 
its  disadvantages  and  is  cumbersome.  Therefore 
it  is  strongly  recommended  not  to  purchase  an 
instrument  of  this  kind  if  it  can  be  avoided. 


The  rack  &&&  pinion  adjustment  is  by  far  prefer- 
able in  every  respect  and  has  stood  the  test  of 
many  years,  although  efforts  have  been  made  to 
introduce  other  methods,  all  of  which,  however, 
have  become  obsolete.  To  be  satisfactory  and 
lasting,  it  must  be  exceedingly  well  made  and  it  is 
safe  to  advise  that  any  instrument  with  this  adjust- 
ment, which  does  not  work  well  at  the  outset  may 
be  regarded  as  a  poor  one.  In  late  years  the 
pinion  with  spirally  cut  teeth  and  the  rack  with 
diagonal  ones  has  come  into  common  use  and  is 
better  than  the  older  form  of  straight  cut  teeth. 
In  order  to  make  the  pinion  operative,  bearings 
are  provided  for  it  in  the  arm  and  its  teeth  engage 
in  the  rack,  which  is  fastened  to  the  slide,  which 
also  has  its  bearing  in  the  recessed  vertical  length 
of  the  arm,  as  shown  in  Fig.  ^(>. 

This  adjustment  must  meet  the  following  con- 
ditions and  if  it  does  not,  the  instrument  may  be 
safely  condemned  as  faulty. 


64 

It  must  work  with  the  utmost  smoothness  and 
with  not  the  least  perceptible  jar  or  grating*. 


lii 

pi,    «  4J 


! 


65 

It  must  be  free  from  lost  motion  when  working 
with  the  highest  powers. 

The  slide  must  be  so  perfectly  fitted,  that  it 
shall  show  no  play  when  the  tube  is  moderately 
forced  from  one  side  to  the  other. 

Fine  Adjustment. — While  this  is  constructed 
in  numerous  ways  it  depends  in  all  upon  a  screw 
for  the  propelling  power.  It  is  sometimes  called 
the  slow  motion,  as  one  revolution  of  the  screw 
seldom  gives  more  than  -£$  inch  motion.  This 
screw  is  also  called  the  micrometer  screw.  The 
brass  button  is  called  its  head  and  when  it  is  pro- 
vided with  equal  divisions  upon  its  upper  surface, 
it  is  the  graduated  head.  In  this  case  a  stationary 
index  is  fastened  to  the  arm.  A  fine  adjustment 
which  will  not  fulfill  the  following  conditions  may 
be  considered  as  faulty. 

The  screw  must  work  freely  and  smoothly  and 
without  any  side  motion  or  play. 

The  adjustment  should  act  promptly  in  the 
forward  and  backward  motion  without  the  least 
particle  of  hesitation  or  lost  motion. 

There  should  not  be  the  slightest  displacement 
of  the  object  in  the  field  when  the  screw  is  worked 
forward  and  backward. 

While  the  fine  adjustment,  even  more  than  the 
stage,  will  show  displacement  with  moderate 
magnifying  power  by  a  slight  exertion  of  force 


66 

against  the  tube,  it  should  return  to  its  position 
when  released. 

If  a  new  instrument  does  not  meet  the  con- 
ditions here  set  down  for  testing  a  fine  or  coarse 
adjustment,  it  may  be  put  down  as  of  faulty  con- 
struction, no  matter  by  whom  made  or  how  well 
made  it  may  appear  in  other  respects. 

Draw-Tube. — While  this  part  of  the  instrument 
may  be  an  advantage  when  judiciously  used,  it 
may  have  a  pernicious  influence  when  abused.  It 
will  give  both  short  and  long  tube  standards  and 
should  be  provided  with  a  mark  to  indicate  each 
length,  or  should  have  divisions  by  which  the 
standard  can  be  read  off.  It  should  not  be  over- 
looked, that  when  a  double  nose-piece  is  used  the 
heighth  of  this  is  added  to  the  optical  length  and 
suitable  allowance  should  be  made  for  it.  In  the 
cheaper  instruments  the  draw-tube  slides  in  the 
inner  surface  of  the  outer  tube,  but  in  the  better 
instruments,  a  special  sleeve  is  provided  in  which 
the  draw-tube  operates.  As  both  of  these,  have 
the  defect  incident  to  the  sliding  tube  adjustment, 
a  cloth  lining  is  preferable  as  the  movement  is 
smooth,  while  firm  and  will  remain  so  for  an 
unlimited  time.  Care  should  be  used  in  moving 
the  draw-tube  as  a  too  sudden  movement  upward 
may  draw  the  main  tube  with  it  and  thus  injure 
the  rack  and  pinion,  or  downward,  may  force  the 
objective  onto  the  object  or  by  the  compression  of 


67 

air  in  the  tube,  may  force  out  the  eyepiece.  To 
slide  it  properly,  hold  the  main  tube  with  one  hand 
and  with  the  other  grasp  the  draw-tube  and  move 
it  up  or  down  by  spiral  movement. 

The  draw-tube  may  be  used  to  vary  the  magni- 
fying power,  but  unless  used  judiciously  may  be 
the  cause  of  more  harm  than  good.  While  this 
feature  will  again  be  touched  upon  in  another 
chapter,  showing  the  optical  effect,  it  will  suffice 
at  the  present  to  state  that  it  should  be  used  only 
with  objectives  of  low  power  or  when  with  high 
powers,  unly  under  well  defined  conditions. 

The  draw-tube  usually  has  at  its  lower  end  a 
diaphragm  to  prevent  reflection  from  the  inner 
surfaces  of  the  tubes  and  this  is  sometimes  also 
utilized  as  a  society  screw  for  attaching  some  very 
low  power  objectives  or  accessories. 

Base. — The  judicious  form  and  weight  of  the 
base  adds  greatly  to  the  stability  of  the  micro- 
scope and  it  is  a  too  common  fault  that  in  many 
instruments,  even  from  reputable  makers,  this 
essential  feature  is  sacrificed  from  wrong  motives 
of  economy,  portability  or  compactness.  While  it 
can  hardly  be  expected  that  when  the  arm  is 
inclined  to  the  horizontal  position  it  shall  be  firm, 
as  it  is  never  used  in  this  position  except  for 
photography  and  must  then  be  clamped  to  the 
table,  it  is  but  reasonable  to  demand  that  when 
the  instrument  is  upright  or  slightly  inclined 


under  ordinary  manipulative  operations,  it  should 
not  be  required  that  the  base  be  held  with  one 
hand,  while  the  other  makes  the  adjustments.  We 
can  imagine  nothing  more  aggravating  than  a  lack 
of  stability.  A  considerable  weight  directly  under 
the  pillar  is  of  little  value,  or  a  great  expansion  of 
the  resting  points  with  extreme  thinness  is  little 
better.  There  should  be  a  combination  of  both 
qualities  and  if  suitable  proportions  are  not  main- 
tained, an  excess  can  hardly  be  called  a  fault, 
whereas  too  little  would  certainly  be. 

A  favorite  and  good  method  to  obtain  increased 
weight  within  reasonable  dimensions  is,  to  load  the 
base  with  lead  and  while  some  dealers,  from  inter- 
ested motives,  point  this  out  as  a  defect,  a  pur- 
chaser need  not  hesitate  on  this  account  if  the 
instrument  is  in  other  respects  acceptable. 

Joint  for  Inclination.  —  This  should  work 
smoothly  but  firmly  and  the  arm  should  remain  in 
any  position  in  which  it  is  placed.  If  it  has  a 
gritty  sensation,  the  two  parts  are  liable  to. "eat" 
and  finally  reach  a  point  where  they  cannot  be 
moved. 

Besides  the  above  qualification  a  good  joint 
should  work  without  the  slightest  back-lash,  when 
the  arm  is  worked  quickly  back  and  forth  over  a 
small  space. 

When  the  arm  comes  against  the  stop  for 
upright  position  it  should  not  lean  forward. 


69 

Mirror  and  Mirror-Bar. — The  proper  illumina- 
tion of  an  object  is  an  important  feature  and 
although  there  are  numerous  accessories  for 
properly  accomplishing  this,  which  will  be  spoken 
of  later  on,  the  mirrors  alone  are  effective  agents 
when  properly  constructed  and  applied,  particu- 
larly when  no  high  magnification  is  used.  The 
plane  mirror  is  generally  used  with  very  low 
powers  and  reflects  light  in  about  its  original  inten- 
sity. The  concave  mirror,  however,  is  intended  to 
concentrate  the  light  so  that  all  the  rays  which 
strike  its  surface  are  reflected  and  come  together 
at  some  point  above  and  the  rays  from  the  surface 
being  contained  within  a  comparatively  small 
space,  cause  an  increased  intensity.  This  point  is 
called  the  focal  point  and  is  usually  arranged  to 
coincide  with  the  opening  of  the  stage,  when 
parallel  rays  such  as  from  the  sky  are  used. .  When 
the  source  of  light  comes  considerably  nearer  to 
the  mirror,  as  for  instance  from  a  lamp  and  the 
rays  are  diverging,  the  focal  distance  becomes 
considerably  longer.  Some  of  the  intensity  is  lost 
in  consequence,  as  well  as  the  degree  of  conver- 
gence. For  this  reason  some  mirror-bars  are  so 
arranged  that  the  distance  of  the  mirror  from  the 
stage  may  be  varied  to  accommodate  the  variation 
in  the  location  of  the  source  of  light.  While  this 
is  of  considerable  aid,  there  is  in  some  instruments 
not  sufficient  room  for  a  complete  accommodation, 


70 

with  the  result  that,  under  certain  conditions,  the 
utmost  effectiveness  of  the  microscope  is  not 
obtained. 

Diaphragm. — This  is  provided  for  regulating 
the  amount  of  light.  While  the  mirror  should 
work  to  its  utmost  capacity,  it  very  often  occurs 
that  for  certain  investigations  a  profuseness  of 
light  is  more  harmful  than  otherwise.  When  too 
much  light  exists,  objects  are  said  to  be  drowned 
in  it  and  thus  often  makes  it  impossible  to 
determine  structure.  An  intelligent  use  of  the 
diaphragm  is  of  great  service. 

Besides  the  revolving  diaphragm  there  are  other 
forms  which  maybe  said  to  be  better — for  instance, 
the  so-called  cap  diaphragms,  which  require  a  sep- 
arate piece  for  each  aperature  and  which  are  held 
by  a  special  substage  receiver;  then  the  dome 
diaphragm,  which  is  a  new  application  of  the  ordi- 
nary revolving  diaphragm.  It  consists  of  a  sub- 
stage  fitting  having  a  dome,  to  which  is  fitted  a 
curved  revolving  diaphragm. 

The  ideal  regulator  of  light  is  the  iris  diaphragm 
consisting  of  a  series  of  overlapping  blades  placed 
around  a  central  opening,  size  of  which  may  be 
varied  by  means  of  a  lever  or  milled  edge.  In  the 
ordinary  revolving  form  the  aperture  is  of  neces- 
sity at  some  distance  from  the  object  and  does  not 
fully  control  the  light  on  account  of  the  stray  rays, 
which  the  other  three  forms  accomplish.  The 


71 

distance  of  diaphragm  from  the  object  is  one  of 
considerable  importance.  The  best  position  is  just 
below  the  surface  of  the  stage,  but  as  this  is  not 
always  possible,  it  should  be  as  near  as  conditions 
will  permit.  The  nearer  it  is,  the  more  intensity 
will  be  maintained  with  suitable  moderation  by 
the  limit  of  opening.  Very  recently  it  has  been 
possible,  to  so  construct  the  iris  diaphragm,  that  it 
passes  up  through  the  opening  of  the  stage  and  is 
flush  with  its  upper  surface. 


OBJECTIVES  AND  EYEPIECES. 

In  taking  up  this  subject  we  would  say  at  the 
outset  that  it  is  fraught  with  difficulties,  as  almost 
all  of  the  features  are  based  on  scientific  facts 
which  can  be  explained  by  mathematical  formulae, 
but  as  it  is  our  purpose  to  give  intelligible  explan- 
ations to  those  who  may  not  be  conversant  with 
algebraic  expressions,  many  of  the  statements  and 
descriptions  will  appear  rather  dog-matic.  We  can 
but  advise  those  who  wish  to  study  the  subject 
further,  to  consult  such  books  as  contain  more 
explicit  information. 

As  has  already  been  said,  the  compound  micro- 
scope is  composed  of  two  lenses,  the  upper  one  of 
which  magnifies  the  image  which  is  formed  by  the 
lower  one.  We  know  that  the  purpose  is  to.  give  a 
greater  magnification  than  can  be  obtained  with 
the  simple  microscope.  The  defects  of  chromatic 
and  spherical  aberration  will  however  become  more 
pronounced  than  they  would  in  the  simple  form, 
to  such  an  extent  as  to  nullify  the  benefit  which 
might  be  derived  from  the  magnifying  power  only. 
In  fact,  magnifying  power  in  itself  is  of  very  little 
value  without  the  attributes  obtained  from  the 


73 

chromatic  and  spherical  corrections  and  their 
qualities  which  will  appear  as  we  proceed.  The 
advent  of  achromatic  lenses  was  the  first  decisive 
step  in  advance  and  has  been  the  foundation  of  all 
later  improvements  and  the  high  standard  of  the 
best  productions  of  the  present  day. 

It  is  a  matter  of  pride  to  Americans  to  note  that 
two  of  our  countrymen,  now  deceased,  were  influ- 
ential in  furthering-  the  progress  to  a  considerable 
extent  and  that  their  memories  should  always  be 
honored  by  the  microscopical  world.  They  should 
be  remembered  with  feelings  of  gratitude,  particu- 
larly as  the  compensation  for  their  efforts  was 
extremely  limited.  The  pioneer  in  microscopical 
optics  in  this  country  was  C.  E.  Spencer,  who  was 
followed  by  R.  B.  Tolles,  and  while  both  men  did 
a  great  amount  of  original  advance  work,  it  is  the 
latter  particularly  who,  by  his  wonderful  achieve- 
ments, created  a  great  discussion  in  European 
circles,  by  enabling  results  to  be  obtained  which  for 
a  long  time  it  was  claimed  could  not  be  accom- 
plished. 

Of  inestimable  value  to  the  scientific  world  have 
been  the  labors  of  that  most  capable  and  genial 
gentleman,  Prof.  E.  Abbe  of  Jena,  to  whom,  while 
best  known  to  the  general  microscopist  for  some  of 
his  more  insignificant  improvements,  such  as  the 
Abbe  condenser  and  Abbe  camera  lucida,  we  are 
much  more  indebted  for  his  profound  disclosure  of 


74 

the  principles  of  microscopical  optics,  as  well  as  to 
the  combined  efforts  of  himself  and  Dr.  Schott  for 
their  labors  in  the  art  of  glass  making  and  to  the 
large  variety  of  glass  which  they  have  placed  at 
the  disposal  of  -opticians,  who  by  this  means  have 
been  able  to  accomplish  much  higher  results  than 
would  otherwise  have  been  the  case.  In  this  con- 
nection it  is  opportune  to  state  that  the  production 
of  this  glass,  generally  termed  Jena  glass,  has  been 
taken  advantage  of  by  unscrupulous  parties  in 
creating  the  impression  that  the  bare  fact  of  using 
this  glass  gives  in  itself  much  better  results.  Such 
is  not  at  all  the  case.  The  merit  of  the  production 
consists  mainly  of  the  large  variety  of  glass  with 
different  ratios  of  refractive  index  and  dispersive 
power. 

As  defects  of  the  objective  have  thus  far  been 
specially  mentioned,  it  will  at  this  point  be  well  to 
state  that  the  single  lens  of  the  eyepiece  has  also 
been  found  unsuited  and  wrhile  an  achromatic  lens 
is  unnecessary,  a  supplementary  lens  below  and 
near  the  upper  lens  has  been  beneficial  in  so 
affecting  the  image,  by  collecting  the  rays,  that  it 
can  be  viewed  at  one  glance  and  without  spherical 
and  chromatic  aberration  which  the  single  lens 
would  show.  It  is  for  this  reason  called  the  field 
lens  or  collective  and  the  upper  lens  the  eye  lens. 
Both  of  these  lenses  are  mounted  so  as  to  form  one 
part  of  the  microscope  with  fixed  relations  and  is 


Fig.  28, 


76 

then  called  eyepiece  or  ocular.  In  the  diagram 
(Fig.  28)  the  course  of  rays  from  the  object  through 
the  objective  and  eyepiece  is  shown.  O  G  repre- 
sents the  objective,  F  L  the  field  lens  and  E  L 
the  eye  lens  of  the  eyepiece.  As  the  rays  from  the 
object  pass  through  the  objective  they  are  seen  to 
cross  before  reaching  the  field  lens,  are  converged 
as  they  pass  through  and  further  converged  by  the 
eye  lens  E  L.  At  the  point  c  d  they  form  a  real 
image  of  the  object,  which  can  readily  be  seen  by 
placing  a  ground  glass  or  piece  of  oiled  paper  at 
this  point.  It  is  an  interesting  experiment  and 
one  which  we  recommend  trying.  The  eye  lens 
enlarges  this  image  and  forms  a  greatly  magnified 
virtual  image  at  e  f.  From  this  diagram  several 
changes  with  consequent  results  can  be  noted. 

If  the  objective  is  of  short  focal  length,  a  larger 
real  image  is  formed  at  c  d. 

If  the  distance  between  objective  and  eyepiece 
is  increased,  an  enlarged  real  image  at  c  d 
results. 

If  the  eyepiece  is  of  higher  power,  an  enlarged 
virtual  image  is  formed  at  e  f. 

In  the  same  manner  a  reduced  magnifying 
power  may  be  obtained  by  reversing  these  con- 
ditions. 

As  has  already  been  stated  a  1  inch  lens  with  a 
distance  of  10  inches  between  it  and  the  image 
gives  a  power  of  10  diameters,  and  the  eyepiece 


77 

multiplies  the  virtual  image  by  the  extent  of  its 
power.  From  this  it  can  be  easily  computed  that 
with  a  1  inch  objective  used  with  a  tube  length  of 
10  inches  and  a  1  inch  eyepiece,  a  magnifying 
power  of  10X10  =  100  will  be  obtained;  or  the 
same  combination  with  a  tube  length  of  5  inches 
will  give  one-half  this  power  or  50. 

Objectives. — These  are  divided  into  two  classes, 
dry  and  immersion.  In  the  dry  there  is  no  inter- 
vening medium  other  than  air  between  the  bottom 
lens  of  the  objective  and  the  top  of  the  cover-glass. 
In  the  immersion  a  liquid  fills  up  this  space.  From 
this  fact  it  is  easily  seen  that  liquid  can  only  be  used 
in  objectives  which  are  quite  close  to  the  cover  and 
therefore  short  focus  or  high  power  and  so  on  the 
other  hand  can  objectives  of  long  focus  or  low 
power  not  be  immersion.  We  will  see  later  that 
the  purpose  of  the  immersion  is  to  obtain  higher 
optical  results,  is  in  fact  a  necessary  condition,  and 
an  objective  which  is  constructed  as  a  dry  one 
cannot  be  used  as  an  immersion,  and  vice  versa. 
Although  there  have  been  objectives  constructed 
which  can  be  used  both  as  immersion  and  dry, 
they  have  gone  into  disuse  as  they  must  suffer 
when  used  in  one  or  the  other  direction  or  in  both. 

Water  was  for  many  years  used  as  immersion 
fluid,  but  cedar  oil  was  discovered  and  as  by  means 
of  this  better  results  are  obtained,  it  has  largely 
taken  its  place.  Its  optical  properties  are  almost 


78 

identical  in  refraction  and  dispersion  with  those  of 
crown  glass  and  it  is  for  this  reason  often  termed 
homogeneous  immersion  fluid,  but  for  brevity  ob- 
jectives which  are  constructed  to  be  used  with  it 
are  called  oil  immersion. 

Tube  Length. — Objectives  are  constructed  and 
their  aberrations  corrected  for  the  length  of  tube 
with  which  they  will  be  used  and  as  has  been  shown 
in  a  previous  chapter  there  are  now  two  generally 
accepted  standards.  They  are  corrected  for  either 
the  long  or  short  tube  and  specially  marked  by  pro- 
gressive firms  and  it  is  hoped  that  this  will  become 
a  universal  custom  in  time.  When  an  objective  is 
not  marked  the  purchaser  should  require  to  know 
the  tube  length  with  which  it  is  to  be  used. 

Nomenclature,   or    Rating  of   Objectives.— 

While  for  many  years  objectives,  or  rather  the 
mountings,  were  marked  arbitrarily  by  the  makers 
and  differently  by  each  maker,  it  is  now  customary 
to  mark  the  objectives  so  that  the  figures  shall 
indicate  the  true  optical  value  ;  on  the  continent 
of  Europe  in  millimeters  and  in  England  and  this 
country  in  inches.  The  obj  ectives  are  rated  accord- 
ing to  a  single  lens,  which  the  combined  value  or 
equivalent  focus  of  the  lenses  contained  in  the 
objective,  shall  equal.  This  is  also  true  of  eye- 
pieces. So,  if  two  combinations  equal  in  magnify- 
ing power  a  single  lens  of  1  inch  focus  they  are 


79 

marked  1  inch,  or  if  a  collection  of  four  lenses  will 
be  equal  to  a  lens  of  T^  inch  focus  it  is  so  marked. 
The  powers  increase  in  proportion  to  the  decrease 
in  focal  length,  so  that  a  ^  for  instance  will  give 
a  real  image  twelve  times  larger  than  1  inch  or  a 
real  magnification  of  120  -times. 

Powers.  —  According  to  their  powers,  objectives 
are  called  low,  medium  or  high  and  are  classified  by 
Carpenter  as  follows  : 

Low  powers,  3  inch,  2  inch,  \\  inch,  1  inch,  f 
inch,  |-  inch. 

Medium  powers,  \  inch,  -fa  inch,  J-  inch,  \  inch. 

High  powers,  \  inch,  \  inch,  -fa  inch,  y1^  inch,  T^ 
inch,  -fo  inch. 

It  might  be  stated  that  such  powers  as  -fa  and  -fa 
inch  are  very  rarely  constructed  at  the  present 
time  and  that  the  -^  inch  may  be  considered  the 
maximum,  although  seldom  used.  The  -fa  inch  is 
the  highest  which  is  ordinarily  used  and  will  give 
all  the  optical  advantages,  while  the  higher  powers 
involve  so  many  mechanical  difficulties  as  to 
increase  the  cost  of  production  very  considerably 
and  as  a  rule  detract  from  the  optical  qualities. 
A  modern  objective  of  the  highest  capacity  may 
be  considered  a  work  of  art  and  there  are  few  pro- 
ductions of  the  human  hand  which  exact  so  much 
untiring  application,  ingenuity  and  skill. 

Systems.  —  An  objective  is  said  to  consist  of 
systems  which  may  vary  in  number  from  one  to 


OF  THB 

I  UNIVERSITY 
V  — 


82 

extent  of  which  is  expressed  in  degrees  and  of  all 
the  qualities  in  an  ideal  objective,  this  is  the  most 
important.  Thus  in  Fig-.  30.  D  is  considered  the 
point  of  focus,  and  c  D  E  the  angular  aperture. 
The  above  definition  has  its  limitations  however. 
While  in  objectives  of  proper  construction  it  holds 
true,  there  are  many  in.  which  it  is  not  the  case. 
For  instance,  an  objective  may  be  so  constructed 
that  it  may  transmit  a  considerable  number  of 
rays  in  excess  of  those  which  combine  to  form  an 
image  and  it  is  evident  that  as  they  do  not  aid  in 
forming  an  image,  they  serve  no  purpose  and 
therefore  have  no  value  in  consideration  of  ang- 
ular aperture. 

As  there  are  many  objectives  of  the  same  power 
but  of  different  angular  aperture,  there  are  again 
others  of  varying  power  but  of  the  same  angle. 

Light  is  radiated  by  an  object  equally  in  all 
directions  aiid  the  more  of  the  rays  which  can  be 
collected  to  form  an  image,  the  more  distinct  will 
it  become  and  to  a  greater  extent  can  we  see  detail. 
If  in  two  objectives  one  receives  on  its  front  sur- 
face and  transmits  a  larger  number  of  rays  than 
another  of  equal  power,  we  have  a  case  where 
power  would  indicate  that  we  should  see  equally 
well,  but  we  will  find  that  there  is  a  difference, 
due  to  the  amount  of  angular  aperture,  in  favor  of 
the  wider  angle.  Or,  in  the  case  of  two  objec- 
tives in  which  one  has  one-half  the  power  of  the 


88 

other,  but  which  takes  in  the  same  amount  of 
rays,  it  would  appear  that  if  the  power  alone  were 
to  indicate  the  visibility  of  the  object,  the  higher 
one  should  show  more  detail,  whereas  they  in 
reality  show  equally  well. 

Aperture,  without  the  defining-  word  angular, 
indicates  a  very  important  feature  in  an  objective 
and  designates  the  beam  or  pencil  of  light  which 
passes  out  through  the  rear  lens  of  an  objective, 
or  in  other  words  is  the  effective  diameter  of  the 
rear  lens.  Many  objectives  are  made  in  which  the 
rear  lens  is  larger  in  diameter  than  the  beam  of 
rays  which,  coming  from  an  object,  can  be  trans- 
mitted through  it  and  while  not  particularly 
detrimental,  has  no  value  except  perhaps  to  lead 
to  a  wrong  conclusion  in  reference  to  angular 
aperture  when  this  is  measured,  as  the  excess  of 
image  forming  rays,  called  stray  rays,  may  indi- 
cate a  greater  angle  than  the  objective  really 
possesses. 

Cover  Glass. — At  this  point  we  must  introduce 
another  feature  which  we  have  not  yet  considered 
but  which  is  important  in  understanding  this,  the 
most  important  property  of  an  objective.  As  we 
have  shown,  in  describing  the  principle  of  refrac- 
tion, rays  which  fall  from  air  upon  a  surface  of 
glass,  are  bent  out  of  their  course  or  refracted. 
The  cover  glass  which  is  used  on  the  object  during 
examination,  or  for  its  preservation,  although 


84 

extremely  thin,  has  a  marked  influence  on  the 
optical  performance  of  an  objective  and  this  influ- 
ence increases  as  the  magnifying  power  of  the 
objective  increases. 

For  the  purpose  of  illustration,  we  will  imagine 
a  cover  glass  of  considerable  thickness  (Fig-.  31). 


gf-\  a 


f 


altf, 


Fig.  31. 

o  represents  the  source  of  light,  or  in  this  case  the 
object:  While  the  rays  from  it  are  emitted  in  all 
directions,  we  need  only  consider  those  coming 
from  the  upper  half,  and  for  simplicity  we  will 
select  from  them  two  pairs.  As  o  a  and  o  a'  strike 
the  lower  surface  of  the  cover  glass  they  are 
refracted  toward  the  axis  o  i  and  on  their  emer- 
gence from  the  upper  surface  of  the  cover  glass 
are  again  refracted  away  from  the  axis  in  the 
direction  of  b  c  and  b'  c',  which  is  the  original  direc- 
tion of  the  rays.  The  same  action  will  take  place 
with  the  extreme  rays  o  d  and  o  d' ,  which  will 
emerge  as  shown  at  e  f  and  e'  f .- 


AIR. 
I. 


WATER. 
II. 


OIL. 
III. 


Fig.  32. 


If  we  now  bring  the  front  lens  of  an  objective 
close  to  the  cover  glass  we  can  study  its  influence 
and  for  this  purpose  will  use  the  simple  but  intel- 
ligible illustrations  of  Prof.  Gage  to  show  the 
same  phenomena. 

I  shows  a  dry    objective    in    which   the   inter- 
vening medium   between   the  top  of  cover  glass 
and  front  of  objective  is  air. 

II  is   a  water   immersion    objective    in    which 
this  space  is  filled  with  water. 

III  is  an   oil   or   homogeneous   immersion    ob- 
jective in    which   the  space  is  filled   with    oil   of 
cedar. 

From  what  we  have  said  in  regard  to  the  laws 
of  refraction  we  know  that  as  the  medium  becomes 
more  dense  the  refraction  becomes  greater  and 
for  the  same  reason  it  is  clear  that  as  the  differ- 
ence in  density  between  the  two  media  becomes 
less  the  refraction  is  proportionately  less.  As 
water  has  a  greater  density  than  air,  but  less  than 
glass,  refraction  between  these  two  media  is  less 
than  between  air  and  glass.  As  the  homogeneous 
fluid  has  the  same  density  as  glass,  or  as  we  have 
stated,  is  virtually  fluid  glass,  no  refraction  between 
these  two  media  takes  place. 

If  we  now  refer  to  the  diagrams  and  will  bear 
in  mind  the  course  of  rays  from  an  object  through 
a  cover  glass,  we  can  follow  out  their  action.  In 
the  case  of  a  dry  objective,  we  see  that  some  of 


the  emerging-  rays  coming  from  the  top  of  the 
cover  glass  are  so  refracted  that  they  fail  to  touch 
the  front  lens  of  the  objective  and  are  therefore 
lost.  In  the  water  immersion,  however,  the  rays 
being  less  bent,  more  of  them  reach  the  front  lens 
and  are  passed  through  the  objective.  With  the 
oil  immersion  the  only  refraction  is  that  which 
takes  place  at  the  lower  surface  of  the  cover  and 
from  that  point  the  rays  are  carried  uninterruptedly 
through  the  cover,  fluid  and  front  lens  as  far  as 
the  convex  surface,  where  they  are  refracted  and 
carried  through  the  objective.  From  a  view  of 
the  diagram  it  might  appear  that  if  the  lens  were 
enlarged  in  diameter  more  of  the  extreme  rays 
which  are  lost  might  be  utilized,  but  as  the  radius 
of  the  front  lens  would  have,  to  be  increased  we 
know  that  its  focus  and  magnifying  power  would 
be  decreased.  This,  however,  would  be  detrimen- 
tal, inasmuch  as,  according  to  the  law  which  has 
been  determined  and  promulgated  by  Prof.  Abbe, 
the  capacity  of  an  objective  is  determined  by  the 
ratio  between  its  focal  length  and  the  diameter  of 
the  emergent  pencil  at  the  point  of  its  emergence, 
from  the  back  of  the  objective,  or  in  other  words, 
it  is  "  the  sine  of  half  the  angle  of  aperture  multi- 
plied by  the  refractive  index  of  the  medium 
between  front  of  objective  and  cover."  This  has 
been  called  by  Prof.  Abbe  the  Numerical  Aperture, 
and  is  now  commonly  used  to  designate  the  effici- 


ency  of  an  objective.  When  applied  to  objective 
mountings  or  used  in  tables  it  is  abbreviated  to 
N.  A.  The  formula  by  which  computations  are 
made  is  N.A.=(«  sine  u)  in  which  N. A.  is  numerical 
aperture,  n  the  index  of  refraction  of  front 
medium  and  u  one-half  the  angle  of  aperture. 
Since  the  media  are  air,  water  or  oil  it  is  necessary 
to  know  the  refractive  index  of  each,  which  is  1.0 
for  air,  1.33  for  water,  1.5  for  oil  and  for  the  ordi- 
nary crown  glass  the  index  is  the  same  as  for  oil. 

To  illustrate  this  formula  better  we  will  take  an 
example.  A  dry  objective  £  inch  focus  has  an 
angular  aperture  of  100  degrees.  By  reference  to 
a  logarithmic  table  we  find  that  one-half  of  the 
sine  of  100  degrees  is  0.766  and  we  know  that  n, 
the  medium  in  front  of  the  objective  being  air, 
has  a  value  equal  to  1.0.  The  formula  then  is,  in 
figures,  N.  A.=n  or  1.0  X  u  or  0.766=0.766. 

To  make  this  computation  we  have  all  figures 
except  that  of  angular  aperture  at  hand.  This 
must  be  determined  and  the  method  for  accomp- 
lishing it  will  be  given  in  a  succeeding  chapter. 

As  a  rule  the  designation  as  to  power  and 
numerical  aperture  engraved  on  the  mounting  of 
an  objective  from  a  responsible  firm  can  be  relied 
upon  as  being  quite  close,  the  variations  seldom 
being  greater  than  is  incident  to  accurate  human 
handi-work  and  such  variations  as  do  occur,  have 
little  influence  on  the  optical  capacity.  It  is  mani- 


festly  difficult  for  opticians  who  produce  large 
quantities  of  objectives  to  measure  and  suitably 
mark  each  individual  product,  particularly  when 
the  differences  are  at  best  slight. 

It  is  easy  to  learn  the  numerical  aperture  of  an 
objective  after  the  angular  aperture  has  been 
determined,  as  the  various  values  for  different 
angles  have  been  computed  and  are  issued  in 
tables.  Such  tables  can  be  found  in  the  pages  of 
the  Journal  of  the  Royal  Microscopical  Society  and 
in  the  catalogue  of  the  Bausch  &  Lomb  Optical  Co. 

Before  the  time  when  the  numerical  aperture 
was  so  thoroughly  elucidated,  it  was  known  that 
an  increase  of  angular  aperture  gave  higher 
results,  but  just  why  this  was  so  was  not  appre- 
ciated. So  also  was  it  difficult  to  understand  that 
a  dry  objective  of  180  degrees,  a  water  immersion 
of  96  degrees  and  an  oil  immersion  of  82  degrees 
had  the  same  effective  aperture. 

How  to  Measure  Angular  Aperture.— In  in- 
struments of  the  American  type  in  which  the  axis 
of  the  mirror-bar  is  in-  the  plane  of  the  stage  and 
in  which  the  circular  part  is  graduated,  the  mat- 
ter of  measuring  angular  aperture  is  quite  simple. 
It  was  first  recommended  by  Mr.  Tolles  and  has 
been  carefully  worked  out  by  Dr.  George  E. 
Blackham  of  Dunkirk,  N.  Y.  After  the  object 
has  been  focused  upon,  incline  the  body  of  the 
microscope  to  a  horizontal  position,  remove  the 


90 

mirror  with  its  bow  from  the  socket  and  place 
therein,  or  by  rubber  band  attach  to  the  end  of 
the  bar,  a  toy  candle  at  such  height  that  the  flame 
will  be  in  the  optical  axis.  The  mirror-bar  is  now 
swung  to  the  right  and  left  to  such  an  extent  that 
one-half  the  field  shall  be  illuminated,  provided 
the  object  shall  still  be  well  defined,  or  to  such  a 
point  where  the  definition  of  the  object  shall 
appear  impaired  without  regard  to  the  illumina- 
tion of  the  field.  These  limits  will  mark  the 
efficient  beam  of  light  which  passes  through  the 
rear  system  of  the  objective.  As  we  have  stated 
before,  in  some  objectives  the  rear  system  is  larger 
in  diameter  than  the  effective  beam  of  light  trans- 
mitted from  the  object,  which  will  permit  stray 
rays  to  reach  the  eyepiece,  but  which  have  no 
value  in  forming  the  image  and  therefore  are  not 
to  be  considered.  In  instruments  which  have  no 
graduated  mirror-bar  the  matter  becomes  more 
difficult  and  less  accurate.  The  following  is  called 
Lister's  method. 

After  the  objective  has  been  focused,  place  the 
body  of  the  microscope  in  a  horizontal  position 
and  in  front  and  some  distance  from  it,  a  candle 
or  lamp,  if  the  latter,  with  the  narrow  edge  of  the 
wick  toward  the  instrument,  but  level  with  the 
tube.  Move  the  lamp  on  each  side  of  the  axis  to  a 
point  as  described  in  the  foregoing  method.  Indi- 
cate the  position  of  the  center  of  the  lamp  at  each 


91 

extreme  on  the  table  and  beneath  the  instrument 
also  a  point  situated  vertically  under  the  object. 
Connect  these  points  and  with  a  protractor  measure 
the  angle. 

A  very  accurate  method  and  one  which  can  be 
carried  out  on  all  instruments  is  that  suggested  by 
Prof.  Abbe  and  for  which  the  firm  of  Carl  Zeiss 
supply  an  apparatus  which  is  called  the  aperto- 
meter,  which  consists  of  a  semi-circular  disk  of 
glass  having  at  its  straight  edge  a  beveled  surface 
which  reflects  the  light  through  a  perforated  disk 
into  the  objective.  Two  strips  of  brass  which  act 
as  stops  are  placed  on  the  arc  and  thus  indicate 
the  limit  of  aperture,  which  can  be  read  off  on  the 
scale  as  well  as  the  corresponding  numerical 
aperture. 

In  the  foregoing  we  have  gone  to  some  length  in 
stating  the  importance  of  angular  and  numerical 
aperture  and  laid  stress  upon  the  influence  which 
these  factors  have  upon  the  efficiency  of  the 
objective.  We  shall  now  endeavor  to  explain  what 
these  attributes  are. 

Resolving  Power. — First  of  all  qualities  in  an 
objective  is  the  resolving  power,  which  is  the 
ability  to  show  intricate  structure  and  minute 
detail,  it  being  of  course  understood  that  the 
objective  is  properly  constructed  so  that  defects 
shall  not  detract  from  this  quality.  It  is  of  course 
clear,  that  no  matter  how  great  the  numerical 


92 

aperture  may  be,  its  effectiveness  may  be  injured 
by  the  presence  of  chromatic  or  spherical  aberra- 
tions or  defective  work  and  it  is  a  matter  of  no 
uncommon  occurrence  that  in  objectives  of  the 
same  power  and  aperture  there  is  a  noticeable 
difference  in  resolving  power,  or  in  objectives  of 
different  numerical  aperture,  the  one  of  less  aper- 
ture will  have  a  greater  resolving  power  than  the 
other.  If  »we  could  accept  the  statements  of 
makers  as  true  ones,  a  portion  of  this  work  and  a 
vast  amount  of  literature  would  be  unnecessary, 
but  the  writer  has  occasion  to  know  that  this 
is  not  the  case  and  that  there  is  a  constantly 
increasing  danger  and  tendency  to  allow  small 
defects  to  pass,  in  spite  of  the  fact  that  the  general 
efficiency  of  the  microscope  has  increased  in  late 
years. 

To  resolve  a  structure  is  to  make  it  visible  and 
the  resolving  power  is  in  direct  ratio  to  the 
numerical  aperture  and  can  be  mathematically 
calculated.  It  can  be  studied  from  the  aperture 
tables  already  mentioned.  It  will  be  seen  that  an 
objective  with  a  numerical  aperture  of  0.50  will 
show  one-half  as  many  lines  as  one  with  a  numer- 
ical aperture  of  1.0.  This,  it  will  be  seen,  refers 
only  to  the  resolving  power  of  an  objective  and 
makes  absolutely  no  reference  to  its  magnifying 
power.  Now,  as  we  know  that  the  purpose  of  the 
compound  microscope  is  to  give  magnifying  power 


and  that  there  is  certain  structure  which  is  not 
visible,  how  can  we  reconcile  this  with  the  fact 
that  numerical  aperature  only  does  it?  Why,  if 
this  is  true,  should  we  use  a  higher  power  object- 
ive with  a  certain  aperture  in  preference  to  a  lower 
one  of  the  same  aperture? 

The  normal  eye  can  see  about  200  lines  to  the 
inch,  or   structures  which    are   ^-^th   of   an   inch 
apart  and  it  is  therefore  evident  that  any  structures 
under  the  microscope  must  be  separated  at  least 
to  this  extent  in   the  virtual  image,  in  order  that 
they  may  become  visible.     While  we  have  shown 
that  magnifying  power  may  be  increased 
by  increasing  power  of  objective, 
by  increasing  power  of  eyepiece-, 
by  increasing  tube  length, 

we  are  limited  in  the  last  direction  mainly  by 
such  length  of  tube  as  has  been  found  conveni- 
ent in  the  construction  of  the  stand;  in  the  case  of 
eyepiece  on  the  one  hand  by  convenience  in  use 
and  on  the  other  by  the  fact  that  too  great  an  increase 
causes  an  indistinctness,  so  that  although  variation 
in  eyepiece  within  narrow  limits  is  admissible,  we 
are  compelled  to  select  an  objective  which,  with  a 
medium  power  eyepiece,  will  give  such  separation 
to  structure  and  fine  detail,  that  it  will  be  visible 
to  the  eye  without  any  undue  strain. 


94 

Chromatic  Aberration. — Up  to  this  point  we 
have  spoken  of  the  correction  of  chromatic  aberra- 
tion by  suitable  use  of  flint  glass  lenses  and  for  the 
purpose  of  not  making  the  subject  too  complex, 
have  purposely  refrained  from  stating  that  entire 
freedom  from  color  is  impossible  in  the  ordinary 
combinations  of  flint  and  crown  glass.  The  cor- 
rection is  for  only  two  colors  of  the  spectrum,  the 
red  and  violet,  leaving  as  a  residue  the  other  colors 
which  appear  as  green  and  purple.  These  are 
called  the  secondary  spectritm  and  on  bright  objects 
can  easily  be  discerned.  For  all  ordinary  purposes 
it  is  not  prejudicial  to  the  performance  of  an 
objective.  It  becomes  more  pronounced  in  dry 
objectives  of  large  aperture  and  when  high  power 
eyepieces  are  used.  In  properly  corrected  low 
and  medium  powers  of  medium  aperture  and  high 
power  immersion  objectives  it  is  most  noticeable, 
but  certainly  not  to  any  extent  to  be  objectionable, 
except  when  oblique  light  is  used. 

Great  care  should  be  used  in  judging  an  object- 
ive by  its  chromatic  correction  and  one  should  not 
be  led  to  false  conclusions  by  the  amount  of  color 
which  an  objective  shows.  It  has  been  a  common 
experience  with  the  writer  to  have  had  objectives 
complained  of  which  were  properly  corrected  and 
which  were  excellent  in  every  respect  except  that 
they  showed  the  secondary  colors,  so  that  it  may 
safely  be  said,  remembering  that  wide  aperture 


•    95 

involves  a  greater  amount  of  color,  that  an  object- 
ive showing  the  proper  colors  of  green  and  violet 
and  having  proper  resolving  power  may  safely  be 
accepted,  or  in  the  choice  of  objectives  between 
one  showing  no  color  but  having  no  resolving 
power,  the  other  having  color  and  resolving  power? 
the  latter  is  certainly  the  preferable  one. 

When  colors  of  green  and  violet  are  not  suffi- 
ciently pronounced  on  an  object  when  the  mirror 
is  in  central  position,  they  will  become  more 
apparent  when  the  mirror  is  swung  to  an  oblique 
position,  using  for  an  object  a  coarse  diatom 
mounted  dry.  If  the  mirror  is  swung  to  the  left 
the  object  will  be  fringed  on  the  right  side  by 
yellow-green  and  on  the  left  by  violet  color,  or  if 
the  mirror  is  swung  to  the  right  the  conditions  are 
reversed. 

When  the  objective  is  not  properly  corrected  it 
is  said  to  be  chromatically  under-corrected  or  over- 
corrected. 

Under-correction  may  be  judged  by  central 
light,  after  the  object  has  been  focused,  by  slightly 
increasing  the  distance  between  objective  and 
object,  when  the  latter  will  show  a  blue  color  and 
by  decreasing  the  distance  an  orange  color.  Or, 
will  show  over-correction,  if  on  increasing  the  dis- 
tance the  orange  will  appear  and  by  decreasing  the 
distance  the  blue.  The  appearance  will  also 
become  more  pronounced  by  using  oblique  light. 


94 

Chromatic  Aberration. — Up  to  this  point  we 
have  spoken  of  the  correction  of  chromatic  aberra- 
tion by  suitable  use  of  flint  glass  lenses  and  for  the 
purpose  of  not  making  the  subject  too  complex, 
have  purposely  refrained  from  stating  that  entire 
freedom  from  color  is  impossible  in  the  ordinary 
combinations  of  flint  and  crown  glass.  The  cor- 
rection is  for  only  two  colors  of  the  spectrum,  the 
red  and  violet,  leaving  as  a  residue  the  other  colors 
which  appear  as  green  and  purple.  These  are 
called  the  secondary  spectrum  and  on  bright  objects 
can  easily  be  discerned.  For  all  ordinary  purposes 
it  is  not  prejudicial  to  the  performance  of  an 
objective.  It  becomes  more  pronounced  in  dry 
objectives  of  large  aperture  and  when  high  power 
eyepieces  are  used.  In  properly  corrected  low 
and  medium  powers  of  medium  aperture  and  high 
power  immersion  objectives  it  is  most  noticeable, 
but  certainly  not  to  any  extent  to  be  objectionable, 
except  when  oblique  light  is  used. 

Great  care  should  be  used  in  judging  an  object- 
ive by  its  chromatic  correction  and  one  should  not 
be  led  to  false  conclusions  by  the  amount  of  color 
which  an  objective  shows.  It  has  been  a  common 
experience  with  the  wrriter  to  have  had  objectives 
complained  of  which  were  properly  corrected  and 
which  were  excellent  in  every  respect  except  that 
they  showed  the  secondary  colors,  so  that  it  may 
safely  be  said,  remembering  that  wide  aperture 


-     95 

involves  a  greater  amount  of  color,  that  an  object- 
ive showing  the  proper  colors  of  green  and  violet 
and  having  proper  resolving  power  may  safely  be 
accepted,  or  in  the  choice  of  objectives  between 
one  showing  no  color  but  having  no  resolving 
power,  the  other  having  color  and  resolving  power, 
the  latter  is  certainly  the  preferable  one. 

When  colors  of  green  and  violet  are  not  suffi- 
ciently pronounced  on  an  object  when  the  mirror 
is  in  central  position,  they  will  become  more 
apparent  when  the  mirror  is  swung  to  an  oblique 
position,  using  for  an  object  a  coarse  diatom 
mounted  dry.  If  the  mirror  is  swung  to  the  left 
the  object  will  be  fringed  on  the  right  side  by 
yellow-green  and  on  the  left  by  violet  color,  or  if 
the  mirror  is  swung  to  the  right  the  conditions  are 
reversed. 

When  the  objective  is  not  properly  corrected  it 
is  said  to  be  chromatically  under-corrected  or  over- 
corrected. 

Under-correction  may  be  judged  by  central 
light,  after  the  object  has  been  focused,  by  slightly 
increasing  the  distance  between  objective  and 
object,  when  the  latter  will  show  a  blue  color  and 
by  decreasing  the  distance  an  orange  color.  Or, 
will  show  over-correction,  if  on  increasing  the  dis- 
tance the  orange  will  appear  and  by  decreasing  the 
distance  the  blue.  The  appearance  will  also 
become  more  pronounced  by  using  oblique  light. 


Under-correction  will  show  when  the  mirror  is 
swung  to  the  left  and  when  the  object  is  fringed 
with  orange  on  the  left  and  the  blue-violet  on  the 
right.  Or,  over- corrected  when  the  colors  are 
reversed. 

Another  method  is  that  recommended  by 
Naegeli  &  Schwendener  in  their  work  on  the 
microscope.  The  right  half  at  the  front  or  back 
of  the  objective  is  covered  by  black  paper  or  tin- 
foil so  that  only  the  other  or  left  half  remains 
optically  effective ;  take  for  an  object  a  line  or  dot 
of  light  which  can  be  easily  produced  by  blacken- 
ing one  surface  of  a  slide  with  chimney  soot  and 
drawing  a  line  across  it  with  a  point. 

Under-correction  shows  when  the  image  has  on 
the  right  side  a  violet  or  blue  border  and  on  the 
left  a  red  or  orange  colored  border. 

Over- correction  shows  on  the  other  hand  when 
the  left  side  appears  violet  and  the  right  red  or 
orange. 

Spherical  Aberration  and  Cover  Glass. — As 

we  have  stated  there  is  a  residue  of  chromatic  and 
spherical  aberration  in  all  ordinary  achromatic 
lenses  and  objectives.  The  use  of  cover  glass 
influences  the  spherical  correction  and  while  not 
appreciable  to  any  extent  in  low  powers,  it  is  very 
sensible  in  the  medium  and  high  powers.  If  we 
refer  to  Fig.  33  it  will  be  remembered  that  the 
rays  from  the  object  do  not  uninterruptedly  reach 


97 

the  surface  of  the  objective  front  but  are  changed  in 
their  course.  If  we  make  use  of  the  same  illustration 
.and  extend  the  refracted  rays  downward  as  shown 
by  the  dotted  lines  e  c,  f  d,  f  d,  e  c  until  they  meet 
at  the  axis,  these  points  will  be  the  apparant  location 
of  the  object  and  will  appear  to  meet  in  the  planes 
£  a  and  d  b  instead  of  at  o.  To  neutralize  this  con- 
dition the  objective  will  require  to  be  spherically 


Fig.  33. 

under-corrected,  by  which  is  understood  that  the 
marginal  rays  will  eminate  from  a  point  near  the 
objective  and  the  central  rays  at  a  greater  dis- 
tance from  the  objective,  or  as  is  shown  in  the 
•diagram,  appear  to  come  from  exactly  the  same 
points,  which  are  the  apparent  positions  of  the 
object  or  g  c,  h  d,  h  d  and  g'  c. 

If  a  thicker  cover  is  used  the  objective  will  re- 
quire to  be  more  under-corrected,  or  if  a  thinner 


98 

one,  less  under  correction  is  needed,  so  that  if  an 
objective  is  corrected  for  a  definite  thickness  of 
cover,  its  correction  will  be  disturbed  if  greater  or 
less  thickness  be  used,  and  since  resolving"  power 
depends,  as  first  condition,  upon  the  proper  cor- 
rection of  the  two  aberrations,  it  will  be  entirely 
lost  if  the  variation  from  the  normal  thickness  is 
considerable. 

It  will  be  well  to  bear  this  phase  in  mind,  as  it 
has  a  great  influence  on  the  efficiency  of  the  objec- 
tive and  will  appear  repeatedly  in  the  instructions 
to  be  given  later  on. 

There  are  several  methods  by  which  corrections 
may  be  made.  In  objectives  with  fixed  mountings, 
such  as  are  ordinarily  used,  in  which  the  lenses 
have  fixed  relations,  which  correct  the  spherical 
aberration  for  a  definite  thickness  of  cover  glass, 
no  change  can  be  made  in  them  and  therefore 
cover  glass  should  be  used,  which  is  very  close  in 
thickness  to  that  which  was  used  as  a  standard. 
Or,  correction  can  be  made  within  narrow  limits 
by  varying  the  tube  length. 

For  a  thick  cover  the  tube  must  be  contracted; 
for  a  thin  cover  the  tube  must  be  extended. 

Objectives  are  also  constructed  in  which  the  dis- 
tances between  lenses  may  be  varied  and  then  are 
called  adjustable. 

For  thick  covers  the  lenses  are  brought  together  y 
or  the  adjustment  is  closed. 


99 

For  thin  covers  the  lenses  are  separated,  or  the 
adjustment  is  open. 

It  is  a  misfortune  that  cover  glasses  are  not 
made  of  the  same  thickness,  but  the  difficulties  in 
producing  them  of  equal  thickness  are  almost  un- 
surmountable,  and  it  is  also  to  be  deplored  that 
opticians  have  not  agreed  upon  a  definite  thickness 
to  which  they  correct  the  objectives  ;  but  since 
this  is  not  the  case,  all  aid  should  be  given  to  the 
microscopist  to  obtain  the  best  possible  results. 
The  list  herewith  given,  which  has  been  prepared 
by  Prof.  Gage  will  show  the  variations  in  thickness 
used  by  different  opticians  as  standard. 

J.  Green, 

J.  Grunow, 

Powell  &  Lealand, 

Spencer  Lens  Co., 

W.  Wales. 

Watson  &  Sons, 

Klonne  &  Muller, 

E.  Leitz, 
R  wink'elj 

Ross  &  Co, 

Bausch  &  Lomb  Optical  Co, 

c-  Zeiss> 
C.  Reichert, 
Gundlach  Optical  Co, 
W.  &  H.  Seibert, 
R.  &  J.  Beck, 
J.  Zentmayer, 

Nachet  et  Fils, 
Bezu^  Hausser  et  Cie? 

Swift  &  Son. 


mm. 


mm. 
mm. 

mm. 

TV6o-T2oV  mm. 
Y1^  mm. 

To5o-T2(ro  mm- 
mm. 


mm 


mm. 


i  o  _  1  5 
VTJO-  TO  o 


100 

The  writer  may  be  pardoned  for  introducing  at 
this  point  extracts  from  a  paper  which  he  read  a 
number  of  years  ago. 

"  The  cover  glass  may  truly  be  called  a  necessary 
evil  ;  for,  while  absolutely  required  in  microscope 
investigations,  there  is  no  adjunct  to  the  micro- 
scope that  has  been  and  is  productive  of  so  much 
evil,  and  has  retarded  the  utilization  of  benefits 
made  possible  by  the  advance  in  the  construction 
of  objectives,  so  much  as  it. 

"  It  must  be  remembered  that  the  majority  of 
objectives  will  always  be  dry,  and  especially  so 
when  improvements,  which  we  hope  are  still  to 
be  made,  are  accomplished.  It  is  an  unfortunate 
circumstance  that  with  this  class  of  objectives  the 
influence  of  variation  in  thickness  of  cover  glasses 
is  most  apparent  ;  but  since  it  is  so,  we  should,  if 
possible,  provide  an  agency  which,  eliminating  the 
personal  factor  of  efficiency,  will  give,  under  all 
conditions,  results  closely  equal  to  those  under 
which  the  objectives  were  originally  corrected. 

"  It  is  surprising  to  see  how  little  attention  is 
paid  to  this  subject  in  the  large  majority  of 
the  standard  works  on  the  microscope.  Almost 
all  books  give  carefully  prepared  illustrations  and 
descriptions  showing  the  effect  on  the  course  of 
light  of  the  interposition  of  the  cover  glass,  and 
after  giving  conclusive  evidence  of  its  disturbing 


101 

influence,  still,  in  a  general  way,  say  it  is  of  little 
moment.     *     *     * 

"The  system  which  I  have  devised  to  aid  in 
overcoming  these  difficulties  depends  in  the  first 
instance  upon  a  micrometer  for  measuring  the 
thickness  of  cover  glass.  (Fig.  34.) 


Fig.  34. 


102 

"  In  objectives  provided  with  cover  correction 
the  graduation  is  so  arranged  as  to  read  to  T^ ^  mm. 
No  matter  what  the  power  of  objective,  the  num- 
ber gives  proper  correction  for  a  thickness  corres- 
ponding to  it.  Thus,  with  a  cover  glass  of  0.20  mm. 
the  collar  of  such  an  objective  need  merely  to  be 
set  at  20  to  give  the  proper  correction  and,  conse- 
quently, the  best  results. 

All  the  other  scales  give  the  correct  tube 
length  in  inches  and  millimeters  for  covers  corres- 
ponding to  them,  and  in  this  manner  offer  a  ready 
and  definite  means  of  correction.  The  tube  lengths 
required  for  the  thinnest  and  thickest  covers  are  so 
extreme  that  probably  no  convenient  means  for 
obtaining  them  can  be  practically  arranged,  but 
they  can  be  so,  approximately  if  not  entirely.  At 
any  rate,  the  micrometer  will  detect  the  require- 
ments before  using  the  covers,  and  those  deviating 
considerably  from  the  normal  can  be  used  on 
objects  for  use  with  low  powers  only,  in  which 
case  the  effect  will  not  be  very  appreciable. 

"  Iri  this  system  I  do  not  overlook  the  fact  that 
variation  in  tube  length  involves  a  variation  in 
magnifying  power ;  but,  except  in  cases  where 
micrometers  are  used,  I  consider  this  of  secondary 
importance,  as  it  always  is  in  comparison  to  results 
obtained  in  resolving  power. 


103 
"  This  system  involves  four  conditions  : 

First — That  all  cover  glasses  be  measured  before 
using  them,  and  %  that  the  thickness  be  noted  on  the 
preparation. 

Second — That  for  convenience  all  draw-tubes  be 
marked  in  inches  or  millimeters,  or  both. 

Third — That  adjustable  objectives  be  corrected 
according  to  this  scale. 

Fourth — That  the  same  tube  length  and  cover 
glass  thickness  be  used  in  all  original  corrections 
of  objectives." 

Penetration. — Penetrating  power  is  the  quality 
which  enables  us  to  look  into  an  object — to  observe 
different  planes  at  one  time.  In  the  mind  of  the 
writer,  it  is  of  no  special  importance,  or  at  any  rate 
not  as  much  as  is  claimed  for  it,  and  if  desired  is 
easily  attained.  It  depends  upon  magnifying 
power  and  angular  aperture,  and  decreases  with 
the  increase  of  either  of  these.  Objectives  are 
generally  not  constructed  with  any  reference  to  it ; 
it  is  a  natural  consequence  of  certain  conditions. 

Penetration  and  resolving  power  are  antago- 
nistic, or  at  any  rate  in  an  inverse  ratio,  and  can 
only  be  combined  to  a  certain  extent.  In  two 
objectives  of  the  same  power  and  aperture,  one 
cannot  have  penetration  as  a  special  feature  and 
the  other  resolving  power  ;  they  will  be  almost 
similar  in  these  qualities  provided  that  they  are 


104 

similarly  corrected.  However,  if  they  are  not 
similar  in  their  angular  aperture  the  one  of  small 
aperture  will  have  more  penetration  than  the 
other.  In  objectives  of  the  same  angle  but  differ- 
ent power,  the  one  of  low  power  will  have  in  itself 
more  penetration  ;  it  will  be  similar  in  its  action 
to  the  eye,  which,  when  an  object  is  close  to  it,  can 
distinguish  but  one  portion  of  it  distinctly,  while, 
as  its  distance  to  the  eye  is  increased,  can  dis- 
tinguish various  parts  of  it  lying  at  different  dis- 
tances, and  will  finally  see  other  objects  outside  of 
it.  By  looking  at  an  object  at  5  feet  distance,  only 
this  can  be  seen  plainly  ;  but,  at  10  feet,  others 
quite  a  distance  in  front  or  back  of  it  can  be  seen 
as  well. 

Low  power  objectives  have  a  proportionately 
greater  penetrating  power  than  medium  or  high 
powers.  In  an  object  of  considerable  thickness, 
different  planes  can  be  observed  at  one  time  with- 
out focusing  on  them  and  thus  obtain  an  apprecia- 
tion of  form  which  is  impossible  in  the  higher 
powers,  as  in  these  the  adjustment  for  different 
depths  is  required. 

-Furthermore  the  accommodation  of  the  eye  is  also 
a  factor  as  it  varies  with  different  persons  and 
thus,  to  a  certain  extent,  is  a  matter  of  individuality. 

Flatness  of  Field. — The  field  in  a  microscope 
is  that  portion  which  is  observed  in  the  eyepiece, 
and  its  flatness  may  be  observed  when  focused  on 


105 

a  flat  object — preferably  a  micrometer.  It  is  said 
to  be  flat  when  all  portions  of  the  object  are  seen 
over  the  entire  field  at  once  without  further  focus- 
ing. When  not  flat,  it  will  be  found  that  as  the 
image  approaches  the  edge  of  the  field  it  becomes 
more  and  more  indistinct,  and  that  the  objective 
must  be  correspondingly  adjusted  ;  in  many  cases 
it  remains  indistinct  or  blurred,  and  this  may  be 
considered  the  most  serious  fault.  In  the  case  of 
looking  at  straight  parallel  lines,  such  as  in  a 
micrometer,  they  will  appear  to  become  more 
curved  as  they  near  the  edge,  as  shown  in  Fig.  35, 


Fig.  35. 

Flatness  of  field  mainly  depends  upon  the  cor- 
rection of  the  spherical  aberrations  and  as  under 
the  best  conditions  the  latter  cannot  be  entirely 
eliminated,  it  is  impossible  to  attain  absolute 
flatness.  It  may  also,  however,  be  due  to  a  faulty 
eyepiece  ;  in  this  case  it  can  be  determined,  by 
observing  whether  it  shows  equally  in  different 
objectives.  With  beginners,  especially,  it  is  usually 
most  complained  of,  owing  probably  to  the  fact 


106 

that  it  is  most  easily  noticeable.  It  is  a  desirable 
quality  and  indicates  to  a  considerable  degree  the 
quality  of  objectives.  It  is  impossible  to  obtain 
absolute  flatness  of  field,  in  objectives  of  sufficient 
angular  aperture  to  meet  the  requirements  of  the 
present  day. 

Working  Distance. — This  term,  strictly  con- 
sidered, is  an  invariable  quality  of  the  objective 
and  is  the  distance  between  the  front  lens  in  the 
objective  and  the  object,  when  the  objective  is  in 
focus  and  is  corrected  for  that  object.  All  objec- 
tives require  a  certain  amount  of  projecting  metal 
to  protect  the  front  lens  and  this,  with  a  certain 
thickness  of  cover  glass,  lessens  the  working  dis- 
tance. In  objectives  with  fixed  mountings  this 
may  be2  and  with  thick  cover  glasses  is,  consider- 
able. It  is  comparatively  unimportant  however, 
for  the  working  microscopist  to  know  the  working 
distance  .per  se  of  his  objectives,  but  of  considerable 
moment  to  know  what  is  the  actual  space  between 
the  objective  and  cover  glass. 

In  objectives  of  low  and  medium  power,  it  is  of 
little  consideration  ;  but  where  it  must  be  ex- 
pressed in  y^-Q  or  yo^-Q  mcn>  it  becomes  a  matter  of 
importance. 

Working  distance  is  spoken  of  as  being  long  or 
-short  and  varies  with  the  power  and  angular  aper- 
ture. Generally  the  working  distance  decreases 
with  the  increase  in  numerical  aperture  and  be- 


107 

comes  greater  as  the  aperture  becomes  smaller.  It 
was  for  a  long  time  considered  that  these  two 
qualities  varied  according  to  a  fixed  rule,  but  this 
at  the  present  time  is  not  considered  to  be  the 
case.  While  in  objectives  of  the  same  aperture  it 
may  vary  considerably,  it  may  in  others  of  differ- 
ent aperture  be  so  that  the  higher  one  may  have 
the  greater  working  distance.  The  skill  of  the 
optician  must  to  a  considerable  degree  determine 
the  amount  of  it. 

It  will  be  seen  from  the  above  that  working  dis- 
tance stands  in  no  direct  relation  to  the  focal 
distance  of  the  objective  and  it  may  be  added,  that 
it  is  never  as  great  as  the  focal  distance  of  a  single 
lens  of  the  same  magnifying  power. 

As  may  be  imagined,  there  is  a  variety  of 
opinions  as  to  what  constitutes  long  or  short  work- 
ing distance  in  a  certain  objective.  No  definite 
rule  can  be  laid  down  for  this,  as  it  is  conditioned 
by  the  skill  and  requirements  of  the  manipulator. 
It  has  several  times  occurred  in  the  experience  of 
the  writer,  that  objectives  were  complained  of  as 
having  no  working  distance  (that  the  objective 
could  not  be  focused)  when  on  investigation  it  was 
found  that  window  glass  or  a  slide  had  been  used 
for  a  cover. 

To  Measure  Working  Distance. — The  actual 
or  available  working  distance  is  of  little  moment 
except  in  objectives  of  medium  or  high  power  and 


108 

then  for  two  reasons.  As  we  will  show  later  how 
to  focus  an  objective,  it  may  be  of  value  to  the 
student  to  take  no  risk  in  endangering-  the  object 
and  in  the  case  of  oil  immersion  objectives,  where 
the  working  distance  is  exceedingly  small,  to  know 
what  thickness  of  cover  glass  may  be  used.  In 
instruments  having  a  graduated  micrometer  screw, 
bring  the  front  of  the  objective  just  in  contact 
with  the  top  of  the  cover  glass.  This  can  best  be 
done  with  no  danger  to  either  objective  or  cover 
glass,  by  grasping  the  instrument  beneath  the 
base  and  raising  it  so  that  the  cover  glass  is  level 
with  the  eye.  By  looking  toward  a  window  the 
slightest  space  can  be  seen.  In  the  case  of  a  heavy 
instrument  lower  the  head  to  the  level  of  cover 
glass.  Note  the  division  and  by  an  upward  focus- 
ing turn  of  the  screw,  bring  the  object  in  focus  and 
read  off  the  distance  traversed.  In  the  case  of  an 
oil  immersion  objective  follow  the  same  process  by 
bringing  the  objective  in  contact  with  the  cover 
glass  dry,  then  separate  slightly  and  inclining  the 
instrument  allow  some  of  the  oil  to  drop  into  the 
space,  then  focus  and  read  off. 

In  the  case  of  a  micromenter  screw  without 
graduations  the  matter  becomes  more  difficult,  but 
can  be  done  in  the  same  manner  although  not  so 
accurately,  by  marking  the  milled  edge  of  the  head 
of  screw  with  a  wax  pencil  or  ink,  in  the  two  posi- 
tions, by  taking  the  middle  of  the  arm  as  a  fixed 


109 

point  and  by  knowing  or  determining"  the  pitch  of 
screw  ascertain  the  value  of  space  traversed. 

It  must  however,  be  borne  in  mind  that  cover 
glass  of  normal  thickness  be  used,  particularly  in 
'oil  immersion  objectives,  since  if  unusual  thickness 
is  used,  working  distance  may  be  entirely  elimi- 
nated. 

Magnifying  Power. — This  is  a  question  of  vital 
importance  in  a  microscope,  not  so  much  as  a 
quality  in  itself,  as  in  connection  with  the  resolving 
power.  The  inquiry  should  not  be  simply,  how 
many  diameters  an  instrument  will  magnify,  but 
what  the  precision  and  extent  of  its  definitions  are 
under  a  certain  magnifying  power.  If  a  high 
magnifying  power  is  all  that  is  desired,  this  may 
be  obtained  to  an  almost  unlimited  extent  by  means 
of  simple  lenses  which  may  be  procured  at  a  small 
pecuniary  outlay  ;  but  these  do  not  give  a  distinct 
image  nor  do  they  make  structure  visible  which, 
be  it  remembered,  it  is  the  purpose  of  the  micro- 
scope to  do. 

The  normal  eye  can  distinguish  from  200  to  250 
lines  to  the  inch  and  in  a  microscope  such  magni- 
fying power  should  be  used  as  will  apparently 
bring  the  structure  which  is  sought  after  at  least 
up  to  this  figure.  To  illustrate  take  a  J  inch  objec- 
tive of  0.77  N.  A.  and  a  2  inch  eyepiece.  An 
objective  of  this  kind  properly  corrected,  resolves 
the  test-object  Pleurosigna  angulatum,  in  which  the 


110 

lines  average  60,000  to  the  inch.  With  the  above 
eyepiece  it  is  utterly  impossible  to  see  them,  while 
if  it  is  replaced  by  a  f  inch  or  1  inch  eyepiece,  they 
can  easily  be  distinguished.  This  is  not  owing  to 
any  peculiar  quality  of  the  eyepiece,  but  merely  to" 
the  fact  that  by  increasing  the  magnifying  power, 
the  dimensions  of  the  object  have  been  increased 
to  such  an  extent  that  the  lines  have  apparently 
been  separated  and  become  visible  to  the  eye. 

Beginners  as  a  rule  are  apt  to  use  too  much 
magnification  or  amplification,  and  often  attempt 
to  view  a  large  surface  with  an  objective  which 
will  show  but  a  small  part  of  it.  It  must  not  be 
forgotten  that  the  apparent  field  of  view  is  de- 
creased as  higher  powers  are  used  and  that  a 
low  power  will  give  a  better  impression  of  a  large, 
coarse  object  and  its  relative  parts,  because  it 
makes  a  larger  surface  visible. 

In  objectives  of  the  same  power,  but  of  different 
angular  apertures,  the  magnifying  power  and  field 
^v^ll  always  be  the  same. 

The  following  table  will  probably  be  of  assist- 
ance to  the  beginner.  After  he  has  become  better 
acquainted  with  his  instrument  his  judgment  will 
dictate  to  him  what  to  do. 

A  power  of  25  diameters  will  show  a  surface  of 
about  \  inch  diameter. 

A  power  of  50  'diameters  will  show  a  surface  of 
about  1^  inch  diameter. 


Ill 

A  power  of  100  diameters  will  show  a  surface  of 
about  -g*Q  inch  diameter. 

A  power  of  500  diameters  will  show  a  surface  of 
about  yJo-  inch  diameter. 

A  power  of  1000  diameters  will  show  a  surface 
of  about  ^i-Q-  inch  diameter. 

This  table  is  approximately  correct  with  a 
Huyghenian  eyepiece. 

As  we  have  already  shown,  magnifying  power 
may  be  obtained  by  increase  of  power  in  objective,, 
eyepiece,  or  increase  of  tube  length  and  have  also 
pointed  out  that  the  objective  should  be  relied  upon 
to  obtain  this  increase. 

Objectives  of  the  same  angular  aperture,  but  of 
different  povvers,  will  give  identical  results  by  bring- 
ing them  up  to  the  same  magnifying  power,  unless 
the  difference  is  considerable.  In  both  objectives 
and  eyepieces  the  lenses  decrease  in  size  with  the 
increase  in  power  and  consequently  give  less  light 
and  while  this  one  objection  exists  in  the  objective, 
an  additional  one  occurs  in  the  eyepiece,  in  that 
the  eye  must  be  brought  closer  to  the  eye-lens  and 
must  be  kept  more  strictly  in  the  optical  axisr 
which  at  a  long  sitting,  becomes  fatiguing. 

Choice  of  eyepiece  should  be  determined  by 
requirements  and  individual  preference.  All  re- 
sponsible manufacturers  and  dealers  make  up  such 
outfits  of  stands,  objectives  and  eyepieces,  as  ex- 


112 

perience  has  taught  them  are  most  generally 
useful. 

•  It  is  safe  to  follow  in  all  work  on  recognized 
forms  (objects  of  which  the  structure  is  known) 
the  rule,  not  to  use  a  higher  power  than  is  necessary 
to  properly  study  them. 

Apochromatic  Objectives. — All  objectives  of 
what  may  be  termed  the  ordinary  corrections  have 
a  residual  chromatic  and  spherical  aberration,  the 
former  being  called  the  secondary  spectrum,  but 
Prof.  Abbe  has  in  this  direction  also  effected  a 
notable  improvement,  which  with  a  uniform  cor- 
rection of  the  spherical  aberration,  corrects  for 
three  colors,  thus  resulting  in  a  closer  concentration 
of  image-forming  rays  which,  with  the  greater 
numerical  aperture  made  possible  by  these  con- 
ditions, results  in  a  higher  resolving  power.  It 
has  been  found  however,  that  in  high  powers  of 
wide  aperture,  even  with  these  objectives,  there  is 
an  outstanding  error  which  by  itself  cannot  be  cor- 
rected. This  is  balanced  in  the  eyepiece,  which  is 
correspondingly  over-corrected  and  is  called  the 
.compensating  eyepiece.  It  has  therefore  been 
necessary  to  make  the  low  powers  under-corrected 
as  well,  so  that  they  might  be  used  with  the  same 
eyepieces,  for  it  is  evident  that  neither  these 
objectives  with  the  Huyghenian  eyepiece,  nor  the 
compensating  eypiece  with  the  achromatic  objec- 
tives will  give  satisfactory  results. 


113 

The  proper  combination  of  apochromatic  objec- 
tive and  compensating  eyepiece  gives  a  beautiful 
image  with  the  maximum  of  resolving  power, 
unapproached  by  any  of  the  achromatic  combina- 
tions. They  furthermore  have  the  advantage  that 
they  are  exactly  suited  to  photomicrography, 
there  being  exact  coincidence  between  the  photo- 
graphic and  visual  image,  which  in  the  achromatic 
objectives  is  not  the  case,  as  these  must  be  either 
corrected  for  photography,  when  they  are  not  sat- 
isfactory for  ordinary  purposes,  or  an  allowance 
must  be  made  for  the  difference  between  two 
images  in  the  camera,  which  is  very  difficult. 

One  objection  to  the  high  powers  has  been  the 
liability  of  deterioration  in  some  of  the  materials 
used,  which  has  happily  been  overcome  by  Prof. 
C.  S.  Hastings  of  New  Haven,  Conn.,  in  the 
apochromatic  objectives,  which  have  been  com- 
puted by  him  for  the  Bausch  &  Lomb  Optical  Co. 

Eyepiece  or  Ocular. — The  purpose  of  the  eye- 
piece is  so  to  refract  the  diverging  rays  coming 
from  the  objective  that  they  will  reach  the  pupil 
of  the  eye  and  while  the  most  simple  part  of  the 
optical  combination  is,  withall,  an  important  part  of 
it.  In  fact  it  may  be  said  that,  owing  probably  to 
its  simplicity,  it  has  been  neglected  and  very  often 
eyepieces  are  furnished  which  are  not  at  all  com- 
mensurate with  the  quality  of  the  objectives. 


114 

They  are  divided  into  two  classes  : 

Negative  eyepieces  in  which  the  focus  is  within 
the  eyepiece  itself  (at  the  diaphragm). 

Positive  eyepieces,  in  which  the  focus  is  outside 
and  below  the  field  lens  and  which  can  be  used  as 
magnifiers. 

Huyghenian  Eyepieces. — This  is  named  after 
Huyghens,  who  is  said  to  have  first  used  it.  It  is 
a  construction  which  is  in  most  general  use, 
although  made  up  in  a  variety  of  mountings.  It 
is  negative  and  consists,  as  we  have  already  stated, 
of  an  eye  lens,  nearest  the  eye  and  field  lens  or  col- 
lective which  is  the  large  lens  nearest  the  objec- 


Fig.  36. 

tive.  Between  them  and  placed  as  the  focus  of 
the  eye  lens  is  a  perforated,  blackened  disk,  called 
a  diaphragm,  which  limits  the  size  of  field  and 


115 

shows  it  within  a  sharply  defined  border.  It  is 
made  up  in  two  forms  : 

The  English  type  as  shown  in  Fig.  36,  which  has 
a  large  tube  fitting  into  the  microscope  tube  and 
a  neck  or  smaller  tube  which  is  usually  arranged 
with  a  cap  to  slide  over  the  eye  lens. 

The  Continental  type  has  a  straight  tube  which 
drops  entirely  into  the  tube  of  the  microscope  and 
rests  upon  the  mounting  of  the  eye  lens. 

Solid  Eyepiece. — This  is  also  a  negative  eye- 
piece and  is  the  invention  of  the  late  R.  B.  Tolles. 
It  is  called  solid,  from  the  fact  that  instead  of 
being  composed  of  two  lenses,  it  consists  of  one 
piece  of  glass,  which  is  cut  to  a  cylindrical  form 
and  on  the  ends  of  which  the  proper  curvatures 
are  ground  and  polished.  The  diaphragm  is  made 
by  cutting  a  circular  groove  into  the  glass  at  the 
proper  distance  between  the  two  surfaces,  which  is 
then  filled  up  with  an  opaque  pigment.  (Fig.  37.) 


11 

/^ A 

Fig.  37. 


These  eyepieces  are  only  made  in  high  powers, 
as  optical  glass  is  usually  not  of  sufficient  homo- 


116 

geneity  to  make  low  powers,  and  their  cost  would 
be  too  considerable,  without  a  corresponding  ad- 
vantage. They  are  usually  only  made  in  powers- 
of  ^  inch  and  stronger  and  for  this  reason  have 
but  a  limited  use. 

Ramsden  Eyepiece. — This  is  a  positive  eye- 
piece and  is  constructed  of  two  plane  convex  lenses 
with  the  convex  surfaces  toward  one  another. 
They  are  especially  useful  as  micrometer  eyepieces 
in  microscopes  and  telescopes,  as  they  are  used  as 
a  magnifier  on  the  fine  divisions  of  micrometer  and 
at  the  same  time  give  the  virtual  image. 

Periscopic,  Orthoscopic  and  Kellner  Eye- 
pieces.— These  are  also  positive  eyepieces  which. 


c 


Fig.  38. 

are  achromatized  by  making  the  eye  lens  a  doublet, 
or  triplet  and  by  its  correction  permits  the  use  of 
a  larger  and  flatter  field.  (Fig.  38.) 


117 

There  are  a  large  number  of  other  eyepieces 
called  by  a  variety  of  names  which  can  be  found 
in  catalogues,  but  for  which  there  is  very  limited 
use.  They,  therefore,  require  no  special  mention 
in  this  connection. 

Compensating  Eyepiece. — In  this  place  it  is 
.also  proper  to  speak  of  the  new  compensating  eye- 
piece as  made  and  suggested  by  Prof.  Abbe. 
These  eyepieces  compensate  the  residual  errors  in 
the  apochromatic  objectives  and  are  of  no  value 
except  when  used  with  them  and  for  projection,  as 
they  are  highly  over-corrected,  as  can  be  seen  in 
the  edge  of  the  field,  which  has  a  strong  orange 
color,  whereas  the  Huyghenian  has  a  blue  color. 

In  this  list  of  eyepieces  there  is  a  searcher  which 
is  of  low  power  and  intended  to  find  objects.  The 
higher  powers  are  for  general  work,  some  of  them 
being  negative  others  positive.  One  more  kind  is 
the  projection  eyepiece  which  is  intended  for  pro- 
jecting an  image  on  a  photographic  plate,  or  on  a 
screen  or  wall. 

Rating  or  Designation. — Until  within  recent 
years  the  rating  of  eyepieces  was  arbitrary,  each 
maker  following  a  system  of  his  own  and  while 
this  is  still  done,  the  general  method  followed  is  to 
name  them  as  in  objectives,  according  to  a  single 
lens,  which  the  equivalent  focus  of  the  eyepiece 
lenses  will  equal  and  do  this  in  inches  or  milli- 
meters. This  conveys  an  idea  of  the  extent  to 


118 

which  an  eyepiece  magnifies  the  real  image.  Thus 
a  2  inch  or  50.0  mm.  eyepiece  magnifies  five  times, 
one  of  1  inch  or  25.4  mm.  focus  ten  times  and  so  on. 

Flatness  of  Field. —  Although  this  depends 
mainly  upon  the  objective,  the  absence  of  it  may 
be  owing  to  a  faulty  construction  of  the  eyepiece. 
If  it  is  so  prominent  as  to  be  easily  noticeable  and 
to  the  same  degree  with  a  number  of  objectives  it 
may  be  ascribed  to  the  eyepiece.  It  must,  how- 
ever, be  remembered  that  an  absolutely  flat  field 
has  not  yet  been  obtained  ;  it  may  be  closely 
approached  by  decreasing  the  diameter  of  field  to 
less  than  its  normal  size. 

Size  of  Field. — Quite  a  general  but  erroneous 
idea  prevails  that  the  size  of  the  tube  has  an  influ- 
ence on  the  size  of  the  field.  Except  in  eyepieces 
of  very  low  power,  or  with  tubes  with  smaller  than 
usual  dimensions,  this  is  not  true.  .  It  must  be 
remembered  that  a  Huyghenian  eyepiece  admits 
of  a  definite  size  of  field  and  this  is  regulated  by 
the  opening  in  the  diaphragm  ;  the  same  size  of 
opening  is  used  in  all  of  the  same  power,  whether 
it  is  an  eyepiece  with  a  large  or  small  diameter. 

Defects. —As  has  been  stated  many  eyepieces 
are  carelessly  constructed  and  possess  defects 
which  interfere  with  obtaining  a  distinct  image. 
These  defects  do  not  show  easily  in  low  power 
objectives,  but  can  readily  be  seen  with  high 


119 

powers.  The  most  frequently  occurring  fault  is 
the  lack  of  perfect  grinding  and  polishing  of  the 
lens  surfaces.  If  the  former,  it  will  show  itself 
as  spots  in  the  field  and  if  the  latter,  as  a  series  of 
streaks  and  shadows,  usually  circular  in  form  as  if 
the  lens  had  been  wiped  with  greasy  fingers. 
This  may  be  the  difficulty,  and  before  passing 
judgment  the  lenses  should  be  carefully  cleaned. 

Another  defect  may  be  in  the  glass  itself,  in  the 
so  called  striae,  which  will  be  indicated  by  dark 
and  light  streaks  across  the  field. 

Care  must  be  taken  not  to  confound  small  parti- 
cles of  dust  which  are  apt  to  fall  upon  the  field 
lens  and  which  at  times  are  very  prominent  in  the 
field,  with  imperfections  of  the  surface.  These 
can  be  distinguished  from  other  defects  only  by 
wiping,  or  using  a  camel's  hair  brush,  and  even 
with  the  utmost  care  some  particles  are  liable  to 
remain. 

The  eyepiece  often  fits  too  closely  in  the  tube 
and  when  making  observations  with  high  powers, 
a  change  of  eyepiece  is  apt  to  disturb  the  object. 
It  should  enter  without  any  friction  and  still  so 
closely  that  it  drops  slowly  into  its  place  from  the 
compression  of  air  in  the  tube  when  objective  is 
attached. 

Eyepieces  are  now  generally  made  parfocal, 
that  is  the  equivalent  foci  are  made  to  correspond, 


120 


so  that  a  change  in  eyepiece  does  not  materially 
effect  a  change  in  focus  and  no  further  adjust- 
ment than  a  slight  turn  of  the  micrometer  screw 
is  required. 


HOW  TO  WORK. 

To  Attach  Eyepiece. —  As  the  exterior  sur- 
faces of  the  eye  lens  and  field  lens  are  exposed, 
they  are  apt  to  become  dusty  and  should  always 
be  carefully  cleaned  before  use.  If  there  are  two 
or  more  eyepieces,  always  use  the  lowest  power 
first.  Eyepieces  should  be  so  loosely  fitted  that 
they  will  drop  into  the  tube  as  far  as  the  collar  by 
their  oivn  weight.  They  do  this  slowly  when  the 
objective  is  attached,  as  an  airtight  compartment 
is  formed  and  air  to  the  extent  of  the  dimensions 
of  the  eyepiece  must  first  be  expelled  from  the 
tube.  To  fix  in  position  however,  it  may  be  hast- 
ened by  gently  pushing  the  eyepiece  downward, 
but  not  to  such  an  extent  as  to  push  in  the  draw- 
tube,  or  force  down  the  coarse  adjustment.  In 
fact,  care  must  be  used  in  applying  the  eyepiece, 
or  sliding  the  draw-tube,  as  the  objective  may  be 
forced  against  the  object  and  thus  destroy  it,  or 
the  focus  may  be  disturbed. 

To  Attach  Objective.— Take  a  low  power 
objective  first  and  after  it  has  been  removed  from 
the  box,  see  that  its  front  lens  is  clean  ;  elevate 
the  tube  by  means  of  the  coarse  adjustment  so 


122 

that  the  nose-piece  shall  be  at  least  two  inches 
from  the  stage  and  place  the .  upper  end  of  screw 
of  objective  in  the  thread  of  the  nose-piece,  hold 
the  lower  end  between  the  fore  and  middle  fingers 
(left  hand)  palm  upwards,  so  that  it  is  in  line  with 
the  tube  and  gently  pressing  upward,  revolve  the 
objective  with  the  thumb  and  forefinger  of  the 
other  hand  (right  hand)  by  the  large  milled  edge 
at  its  upper  end,  until  shoulder  sets  against 
shoulder.  To  properly  attach  an  objective  is  not 
always  simple  and  cannot  be  done  too  carefully. 
One  danger  lies  in  the  fact  that  the  objective  may 
be  dropped  onto  the  object  and  thus  injure  or 
destroyaone  or  the  other  or  both  and  another  that 
the  thread  is  started  wrong  by  holding  the  objec- 
tive sideways  and  the  threads  may  thus  be 
destroyed. 

In  this  connection  we  draw  particular  attention 
again  to  the  convenience  of  the  double,  triple  and 
quadruple  nose-pieces.  The  convenience  which  is 
obtained  from  their  use,  freedom  from  danger  to 
objective  and  object  and  saving  of  time,  com- 
mend them  in  all  cases  where  two  or  more  objec- 
tives are  used. 

Finding  an  Object. —  The  slide  upon  which 
the  object  is  mounted  is  placed  upon  the  front  of 
the  stage  and  slipped  under  the  two  spring  clips 
to  a  point  where  the  object  c.omes  as  nearly  as 
possible  in  the  center  of  the  opening  of  the  stage. 


123 

The  slide  should  pass  easily  under  the  clips- 
which  however,  is  not  the  case  when  the  edge  of 
the  clips  are  too  bluntly  rounded  and  when  the 
springs  are  too  stiff.  In  either  case  the  clips  may 
be  bent  so  that  the  slide  will  work  easily.  Some 
persons  prefer  to  work  without  clips,  but  this  can 
only  be  done  after  considerable  experience  has 
been  acquired  and  only  when  the  instrument  is  in 
an  upright  position. 

With  the  low  power  objectives,  which  are  used 
on  coarse  and  usually  large  objects,  it  will  be 
found  after  properly  focusing,  that  a  portion  of 
the  object  will  show  itself  in  the  field  and  by  mov- 
ing it,  can  easily  be  brought  to  the  center.  In 
this  connection  it  must  be  remembered  that  the 
image  in  the  eyepiece  is  in  a  reversed  position 
from  that  of  the  object  and  that  a  movement  of 
the  object  to  the  left  gives  an  apparent  movement 
to  the  right  in  the  field.  This  will  create  some- 
confusion  at  the  outset,  but  after  a  little  practice 
the  movement  becomes  involuntary.  In  the  case 
of  a  small  object  which  is  not  found  after  the 
objective  is  known  to  be  in  focus  and  which  is 
easily  recognized  by  the  mounting  medium  or 
small  particles  of  dust  on  the  cover  glass,  move 
the  slide  about  on  the  stage  by  grasping  one  end 
with  the  thumb  and  forefinger,  when  it  can  usually 
be  recognized  by  the  shadowy  outlines  of  the 
object  as  it  flits  across  the  field.  The  difficulty  of 


124 

finding  an  object  or  a  particular  spot  in  it  becomes 
more  difficult  with  the  increase  in  power  and  even 
in  experienced  hands  becomes  quite  vexatious. 
Recourse  may  be  had  to  two  methods  : 

First. —  By  using  a  low  power  eyepiece. 

Second. —  By  using  a  low  power  objective  as  a 
finder. 

The  object  is  thus  found  and  after  moving  to 
the  center  of  the  field  the  objective  is  removed 
and  the  high  power  attached,  or  in  case  a  revolv- 
ing nose-piece  is  used,  swing  the  high  power  objec- 
tive in  position,  care  being  taken  not  to  touch  the 
slide,  and  focus  in  the  manner  to  be  described.  '  A 
low  power  eyepiece  in  this  connection  is  also  very 
useful.  The  object  may  not  be  in  the  field,  due  to 
a  slight  variation  in  the  centers  of  the  objectives, 
but  it  will  certainly  be  very  close  and  ought  to  be 
easily  found. 

The  Mechanical  Stage  in  either  the  fixed  or 
attachable  form  will  be  found  to  greatly  facilitate 
work  in  this  direction,  particularly  if  the  object  is 
minute  and  if  in  an  important  investigation  one 
desires  to  be  absoluely  convinced  that  the  w^hole 
field  of  the  object  has  been  covered,  as  for  instance 
in  the  search  for  bacilli. 

To  Illuminate  the  Object. — This  is  an  ex- 
tremely important  feature  and  should  always  be 
carefully  done,  as  one  may  easily  fail  to  obtain  the 


best  results,  may  be  led  to  wrong  conclusions,  or 
may  injure  the  eyes. 

The  mirrors  of  the  microscope  are  usually  plane 
and  concave.  As  it  is  clearly  inconvenient  or  im- 
possible to  hold  the  microscope  toward  the  source 
of  light,  they  are  provided  to  reflect  the  light 
upward  to  the  object  when  this  is  transparent. 
The  plane  mirror  reflects  the  light  in  the  initial 
intensity  of  its  source  and  is  used  with  low  power 
objectives.  The  concave  mirror  concentrates  the 
rays  on  the  object  and  thereby  gives  intensified 
illumination  and  is  used  with  medium  and  high 
power  objectives,  except  when  substage  condenser 
is  used,  which  subject  is  left  for  future  considera- 
tion. 

The  sources  of  light  are  either  daylight,  or 
artificial  light  from  a  lamp.  In  the  former  the 
light  from  a  northern  sky  is  preferable  and  in  the 
latter  a  flat-wick  oil  lamp,  or  a  Wellsbach  gas- 
flame.  An  ordinary  gas  flame  should  not  be  used 
on  account  of  the  difficulty  of  obtaining  equal 
illumination  and  the  constant  flickering  which  is 
very  injurious.  When  using  the  flat-wick  lamp 
the  narrow  edge  of  the  flame  should  be  used,  as 
this  is  more  intense  than  the  broad  side,  as  can 
be  easily  determined  by  experiment. 

When  using  daylight,  place  the  microscope  as 
near  as  possible  directly  before  a  window  and 
when  a  lamp  is  employed  have  it  on  the  table 
within  easy  reach. 


126 

Light  is  either  transmitted  or  reflected.  When 
the  former,  it  is  used  on  transparent  objects  and 
passes  through  the  objects  from  below  the  stage 
into  the  objective.  In  opaque  objects  this  is 
impossible  and  reflected  light  is  required,  when  it 
is  directed  onto  the  object  from  above  and 
illuminates  its  upper  surface.  In  the  following 
instructions  it  is  assumed  that  transmitted  light 
is  used  unless  otherwise  stated. 

The  concave  mirror  converges  the  light  and 
therefore  has  a  focal  point  and  it  is  evident  that 
if  its  focus  is  of  such  length  that  with  parallel  rays 
(daylight)  it  will  fall  on  the  object,  the  focus  will 
l>e  longer  with  the  diverging  rays  (lamplight)  and 
when  no  provision  is  made  in  the  instrument  to 
adjust  the  mirror  to  meet  these  two  conditions,  it 
becomes  difficult  and  sometimes  impossible  in 
critical  work  to  obtain  the  best  result.  For  this 
and  other  advantages  an  additional  illuminating 
apparatus,  called  a  condenser,  is  now  commonly 
used. 

Before  lighting  an  object  make  certain  that  the 
mirror-bar  is  in  exactly  central  position  and  set 
the  mirror  at  such  an  angle  to  the  light,  that  it  will 
be  directed  upon  the  object,  which  can  be  done 
most  quickly  at  the  outset  by  observing  the  object 
direct,  keeping  the  head  at  one  side  of  the  tube. 
Now  remove  the  eyepiece  and  observe  the  light 
through  the  objective.  It  should  be  central  and  of 


127 

equal  intensity,  which  with  daylight  is  sometimes 
difficult,  as  the  sash  of  the  window  may  be  reflected 
and  show  itself  in  the  field  as  dark  bands,  or  in  the 
case  of  lamplight  the  blue  portion  of  the  flame  may 
appear  as  a  dark  spot.  These  are  only  preliminary 
directions  but  will  suffice  for  the  beginning.  There 
will  be  little  difficulty  in  obtaining  proper  illumi- 
nation at  the  outset,  if  one  will  bear  in  mind  the 
three  necessary  conditions  when  looking  at  the 
back  of  the  objective  : 

Central  illumination, 

Even  illumination  over  the  entire  field, 

Mellow  illumination. 

Defects  in  illumination  which  may  not  be  ap- 
parent will  show  when  the  eyepiece  is  again 
attached,  and  defective  lighting  will  be  indicated, 

When  dark  points  or  shadows  appear  in  the 

field, 
When  a  shadow  is  thrown  at  one  side  of 

the  object, 
When  the   object  appears  to  lie  in  a  glare 

of  light. 

In  the  first  two  cases  the  correction  can  be  made 
by  suitably  adjusting  the  position  of  the  mirror. 
In  the  latter  by  reducing  the  amount  of  light, 
(which  it  may  be  said  in  passing,  is  seldom  neces- 
sary with  medium  or  high  power  objectives),  by 
the  use  of  a  diaphragm. 


128 

When  first  setting  up  the  instrument  the  largest 
opening  of  the  diaphragm  should  be  used  and  if 
the  light  is  found  too  intense,  it  may  be  reduced 
by  bringing  into  position  a  smaller  stop,  or  in  case 
of  the  iris  diaphragm,  by  reducing  the  size 
of  opening.  The  iris  diaphragm  is  infinitely 
superior  to  the  revolving  or  cap  diaphragm  as 
these  have  fixed  openings,  whereas  with  the  former 
any  intermediate  gradation  may  be  obtained. 

It  is  now  generally  conceded  that  observations 
may  be  made  with  the  microscope  to  any  extent 
without  any  detrimental  results  to  the  eyes,  pro- 
vided however,  that  the  conditions  of  light  are  just 
right.  It  is  a  good  rule  to  follow,  to  use  the  least 
illumination  which  will  show  the  structure  being 
studied  and  it  may  also  be  safely  accepted  that  if 
the  eye  tires  or  feels  uncomfortable,  that  the  light 
should  be  moderated. 

Illumination  is  either — 

Central  or  axial,  when  the  center  of  the  mirror 
is  in  the  optical  axis,  or 

Oblique,  when  the  mirror  is  swung  to  one  side 
which,  in  objectives  of  wide  aperture,  will  disclose 
structure  which  cannot  be  seen  with  central  illu- 
mination. 

How  to  Focus. — A  safe  method  to  follow  and 
one  which  is  generally  in  vogue  by  all  careful 
manipulators,  is  to  focus  itpwards — that  is,  bring 
the  objective  closer  to  the  object  than  its  working 


129 

distance  and,  applying"  the  eye  to  the  tube,  elevate 
the  objective  by  the  coarse  adjustment  until  it  is 
in  focus.  In  the  lower  power  objectives,  in  which 
working"  distance  is  considerable,  this  may  appear 
unnecessary,  but  is  nevertheless  strongly  recom- 
mended, principally  to  retain  the  systematic 
method.  In  the  medium  and  high  powers  it  is  very 
important  as  the  working-  distance  is  small.  In 
these,  the  front  of  the  objective  is  brought  down 
nearly  in  contact  with  the  cover  glass  and  the  dis- 
tance should  be  judged  by  holding  the  head  down 
to  the  level  of  the  stage  and  observing  the  distance 
as  the  objective  is  brought  downward.  The  rack 
and  pinion  is  infinitely  superior  to  the  sliding  tube, 
as  the  distance  can  be  traversed  with  absolute  cer- 
tainty without  any  liabilitv  to  slip.  Then  apply 
the  eye  to  the  eyepiece  and  rack  upward,  slowly 
however,  that  the  point  at  which  the  focus  is 
reached  may  not  be  overlooked,  which  may  easily 
be  the  case  in  transparent  and  faintly  colored 
objects.  After  a  better  acquaintance  with  the 
working  distance  of  the  objective  the  proper  dis- 
tance can  be  judged  quite  closely. 

After  the  focus  has  been  found  the  fine  adjust- 
ment should  be  brought  into  action.  This  should 
only  be  used  after  the  focus  has  been  obtained  with 
the  coarse  adjustment,  in  order  to  focus  through 
the  different  planes  or  depths  of  the  object.  Its 
range  of  movement  is  necessarily  short  and  at  one 


130 

end  of  the  screw  comes  to  a  stop  and  at  the  other 
goes  beyond  the  limit  of  the  movement  and  be- 
comes inoperative.  It  should  always  be  kept  as 
near  as  possible  at  the  medium  point.  During 
observation  of  the  object,  the  milled  head  of  the 
fine  adjustment  should  always  be  kept  between  the 
thumb  and  forefinger  of  one  hand  (right)  turning 
the  screw  in  either  direction  to  focus  in  different 
planes  of  the  object,  while  the  other  hand  (left) 
moves  the  object  about. 

Which  Eye  to  Use.— The  right  eye  is  generally 
used  for  observations,  but  while  the  manipulator 
may  from  habit  be  inclined  to  use  this,  it  may  be 
possible  that  in  some  cases  the  left  can  be  used  to 
better  advantage  and  with  less  fatigue.  It  is  a  fact 
well  known  to  oculists  and  opticians  that  many 
eyes  are  defective,  of  which  fact  the  possessors 
may  not  be  aware.  Short  or  long  sightedness  has 
little  or  no  influence  in  viewing  an  object,  except 
to  require  a  different  adjustment,  but  so  called 
astigmatism,  by  which  lines  in  a  certain  axis  cannot 
be  seen  distinctly,  may  influence  the  best  results. 
If  this  error  is  corrected  by  wearing  glasses  and 
these  are  used  while  making  observations,  either 
eye  can  be  used.  But,  in  order  to  determine 
whether  a  defect  exists,  of  which  the  possessor  is 
not  aware,  observe  closely  with  both  eyes,  pre- 
ferably an  object  with  fine  striations,  such  as 
Diatomacae,  to  learn  whether  with  one  eye  a 


131 

better  view  is  obtained  than  with  the  other  and 
use  the  one  giving  the  best  results. 

As  already  stated,  make  it  a  habit  at  the  outset 
to  keep  both  eyes  open. 

There  is  one  point  over  the  lens  called  the  eye- 
point  at  which  the  rays  cross  within  the  smallest 
compass  and  this  is  the  proper  position  for  the  eye, 
as  the  largest  number  of  rays  enter  it.  When 
above  or  below  this  point  the  size  of  field  will  be 
either  reduced,  or  shadows  or  colors  will  appear  in 
it.  In  low  power  eyepieces  the  eye-point  is  farther 
than  the  lens  ;  in  high  powers  quite  close — in  fact, 
in  some  so  close  that  the  eyelashes  may  rest  upon 
the  lens  and  may  sometimes  appear  to  be  in  the 
field  as  dark  bars.  Generally  speaking  the  best 
point  is  where  the  entire  field  is  seen  and  its  mar- 
gin (diaphragm)  sharply  defined. 

What  Objects  to  Use. — Suitable  objects  to 
test  the  capacity  of  objectives  are  also  valuable  in 
leading  the  student  to  a  skillful  use  of  the  instru- 
ment and  in  giving  him  proper  judgment. 

Low  Powers — Proboscis  of  blow-fly.  This  should 
be  flat  and  transparent.  For  1  inch,  f  and  |  inch 
objectives  the  scales  from  Lepisma  sacharina. 

Medium  Powers — Pleurosigma  angulatum,  dry, 
stained  Bacteria  and  Micrococci. 

High  Powers — Oil  immersion  ^  inch  and  -^  inch 
objectives,  Amphiplerua  pelucida,  Surirella  gemma 


132 

in  balsam  or  styrax.     Stained  Bacteria  and  Micro- 
cocci. 

Test  Plate. — This  will  be  an  excellent  acqui- 
sition for  all  those  who  can  meet  the  pecuniary 
outlay.  It  consists  of  a  series  of  20  diatoms  ar- 
ranged according-  to  the  coarseness  of  the  lines. 
They  are  furnished  mounted  both  in  balsam  and 
styrax.  Below  is  a  table  giving  the  numbers, 
names  of  the  various  diatoms  and  divisions  on 
their  surfaces  to  yoVo  mcn-  A  specimen  of  Eupo- 
discus  argus  begins  and  ends  the  series  : 

Stiriae  in  y^ 
inch. 

1.  Triceratium  favus,  Ehrbg.,  -     3.1  to    4. 

2.  Pinnularia  nobilis,  Ehrbg.,  11.7  to  14. 

3.  Navicula  lyra,  Ehrbg.  var.,  -  14.5  to  18. 

4.  Navicula  lyra,  Ehrbg.,  23.    to  30.5 

5.  Pinnularia  interrupta,  Sm.  var.,     -  25.5  to  29.5 

6.  Stauroneis  phoenicenteron,  Ehrbg.,  31.    to  36.5 

7.  Grammatophora  marina,  Sm.,         -  36.    to  39. 

8.  Pleurosigma  balticum,  Sm.,  32.    to  37. 

9.  Pleurosigma     acuminatum     (Kg.) 

Grun.,  -  41.  to  46.5 

10.  Nitzschia  amphioxys,  Sm.,  43.  to  49. 

11.  Pleurosigma  angulatum,  Sm.,         -  44.  to  49. 

12.  Grammatophora  oceanica,  Ehrbg. 

G.  subtilissima,  60.  to  67. 

13.  Surirella  gemma,  Ehrbg.,  -  43.  to  54. 

14.  Nitschia  sigmoidea,  Sm.,  61.  to  64. 

15.  Pleurosigma  fasciola,  Sm.  var.,  -  55.  to  58. 


133 

16.  Surirella  gemma,  Ehrbg.    -  64.    to  69. 

17.  Cymatopleura  elliptica,  Breb.,        -  55.    to  81. 

18.  Navicula  crassinervis,  Breb.  Frus- 

tulia  saxonica,  Rabh.,       - 

19.  Nitzschia  curvula,  Sm., 

20.  Amphipleura  pellucida,  Kg., 

Whatever  opinion  one  may  have  in  reference  to 
the  study  of  diatoms,  the  fact  cannot  be  gainsaid 
that  they  have  been  a  great  aid  in  the  improvement 
of  objectives,  are  used  by  opticians  to  judge  their 
various  characteristics  and  offer  a  reliable  standard 
for  testing  the  resolving  qualities,  such  as  no  other 
objects  can.  The  writer  particularly  recommends 
that  the  test  P.  angulatum,  dry,  form  a  part  of 
every  outfit,  not  so  much  as  a  test  f or  resolvability 
as  to  use  it  for  study  and  to  acquire  quickly  a 
knowledge  how  to  judge  the  phenomena  of  spheri- 
cal aberration.  It  may  be  used  on  powers  £  inch 
and  higher  and  after  it  has  served  its  purpose  may 
be  put  on  one  side  to  work  over  objects  which 
come  under  the  particular  branch  of  study  which 
one  is  following. 

The  writer  wishes  to  counteract  as  much  as  pos- 
sible the  opinion,  which  is  too  prevalent,  that  the 
use  of  diatoms  indicates  microscopical  play  and  is 
unworthy  of  consideration  in  histological  and  bio- 
logical work,  but  the  fact  that  the  optician  deems 
them  necessary  for  determination  of  optical  quali- 
ties ought  at  least  to  indicate  that  they  are  a  valu- 


134 

able    adjunct    and   certainly   will    aid    in    giving 
greater  manipulative  skill. 

At  this  point  it  is  considered  advisable  to  add 
some  suggestions  from  Carpenter. 

"  The  correctness  of  the  conclusions  which  the 
microscopist  will  draw  regarding  the  nature  of 
any  object  from  the  visual  appearance  which  it 
presents  to  him,  when  examined  in  the  various 
modes  now  specified,  will  necessarily  depend  in  a 
great  degree  upon  his  previous  experience  in 
microscopic  observations  and  upon  his  knowledge 
of  the  class  of  bodies  to  which  the  particular 
specimen  may  belong.  Not  only  are  observations 
of  any  kind  liable  to  certain  fallacies  arising  out 
of  the  previous  notions  which  the  observer  may 
entertain  in  regard  to  the  constitution  of  the 
objects  or  the  nature  of  the  actions  to  which  his 
attention  is  directed,  but  even  the  most  practiced 
observer  is  apt  to  take  no  note  of  such  phenomena 
as  his  mind  is  not  prepared  to  appreciate.  Errors 
and  imperfections  of  this  kind  can  only  be  cor- 
rected, it  is  obvious,  by  general  advance  in  scien- 
tific knowledge  ;  but  the  history  of  them  affords  a 
useful  warning  against  hasty  conclusions  drawn 
from  a  too  cursory  examination.  The  suspension 
of  the  judgment,  whenever  there  seems  room  for 
doubt  is  a  lesson  inculcated  by  all  those  philo- 
sophers who  have  gained  the  highest  repute  for 
practical  wisdom  ;  and  it  is  one  which  the  micro- 


135 

scopist  cannot  too  soon  learn  or  too  constantly 
practice.  Besides  these  general  warnings,  how- 
ever, certain  special  cautions  should  be  given  to 
the  young  microscopist  with  regard  to  errors  into 
which  he  is  liable  to  be  led  even  when  the  very 
best  instruments  are  employed." 

How  to  Set  Up  the  Instrument. — Draw  the 
instrument  from  the  case  by  grasping  the  base, 
free  it  from  dust  with  a  large  camel's  hair  brush, 
1  inch  or  1^  inch  wide,  or  by  wiping  carefully 
with  a  chamois  skin,  or  old  linen  handkerchief. 
Place  the  instrument  with  proper  relation  to  light 
on  the  work  table,  which  should  be  of  such  height 
that  observations  can  be  made  with  the  utmost 
possible  comfort  without  straining  the  neck  or 
compressing  the  chest.  Bear  in  mind  always  to 
sit  as  upright  as  possible. 

Bring  the  draw-tube  to  standard  length  by 
drawing  the  draw-tube  to  the  length  for  which  the 
objectives  are  corrected.  Do  this  by  grasping  the 
milled  edge  of  the  draw-tube  and  give  it  a  spiral 
motion  while  holding  the  main  tube  with  the  other 
hand,  which  will  facilitate  its  easy  movement. 
The  objection  being,  however,  that  in  any  but 
cloth-lined  sheaths  the  polished  tube  will  soon  be 
scratched,  especially  if  not  kept  very  clean.  In 
stands  without  the  graduated  tube,  a  mark  or  ring 
is,  or  should  be,  provided  on  it,  which  should  be 
made  to  coincide  with  the  upper  en4  of  main  tube, 


136 

Where  the  graduated  draw-tube  is  provided  bring 
the  proper  figure,  either  216.0  or  160.0  mm.,  in  line 
with  the  upper  end  of  main  tube,  in  accordance 
with  the  tube  length  to  which  the  objectives  are 
corrected. 

Attach  low  power  eyepiece. 

Attach  low  power  objective. 

Place  object  on  stage. 

Illuminate  object. 

Focus  on  object. 

If  the  instrument  is  used  in  the  upright  position, 
place  the  base  close  to  the  edge  of  the  table  ;  if  in- 
clined, it  may  set  farther  in.  Rest  the  arms,  as 
much  as  the  height  of  the  instrument  will  permit, 
upon  the  table. 

How  to  Work. —  It  is  now  supposed  that  the 
instrument  is  ready  for  work.  To  start,  it  is  well 
for  the  beginner  to  provide  a  few  prepared  speci- 
mens, as  these  will  help  him  considerably  if  it  is 
his  intention,  as  it  should  be,  to  prepare  them 
later  himself.  If  only  a  portion  of  the  object  can 
be  seen  and  it  is  desired  to  see  a  larger  surface, 
the  length  of  tube  may  be  contracted  by  means 
of  the  draw-tube.  In  this  case  the  object  will  be 
placed  out  of  focus  and  another  adjustment  be- 
comes necessary.  If  a  higher  power  is  desired 
the  draw-tube  may  be  extended.  Observe  whether 
the  field  is  well  illuminated  and  if  not  bear  in  mind 
what  has  been  said  about  properly  adjusting  the 


137 

mirror.  Now  use  the  micrometer  screw,  remem- 
bering to  grasp  the  milled  edge  between  the 
thumb  and  forefinger,  and  work  to  the  right  and 
left,  to  reach  different  depths,  and  note  carefully 
the  beautiful  structure  which  is  open  to  view. 

Opaque  Object. — We  will  now  suppose  that 
the  object  is  a  mineral  or  plant.  Place  this  upon 
a  slide  and  slip  under  the  clips.  In  this  case  the 
low  power  objective  is  used  for  two  reasons ; 
because  a  general  view  is  sought,  involving  low 
magnification  and  large  field  with  light-giving 
power  and  if  it  should  be  desired  to  use  a  higher 
power  this  cannot  be  done  on  account  of  the  short 
working  distance.  The  light  may  and  undoubt- 
edly will  be  found  insufficient  to  distinguish  the 
object  clearly.  •  If  the  instrument  is  of  the 
American  type,  swing  the  mirror-bar  upon  its  axis 
around  the  stage  to  a  point  above  it  so  that  it  will 
be  at  an  angle  of  about  45°  to  its  surface.  If  a 
lamp  is  used  and  is  in  the  same  position  as  when 
used  for  transmitted  light,  it  is  probable  that  the 
tube  of  the  instrument  will  obstruct  the  light  and 
it  is  then  well  to  move  it  toward  the  front.  Using 
the  concave  mirror,  adjust  it  so  that  the  light  will 
be  concentrated  upon  the  object,  by  watching  it 
directly,  and  then  observe  through  the  tube.  If  it 
is  not  sufficiently  illuminated  continue  to  adjust 
the  mirror  ;  also  vary  its  distance  from  the  object 
and  swing  the  mirror-bar  to  a  higher  or  lower 
point, 


138 

Medium  Power  Objective. —  After  sufficient 
time  has  been  devoted  to  study  with  the  low 
power  objective,  exchange  it  for  the  higher 
power  and  replace  the  object  by  the  slide  P.  angu- 
latum.  Focus  upon  this,  being  mindful  of  the 
suggestions  previously  given  and  do  not  fail  to 
observe  what  has  been  said  in  regard  to  well 
illuminated  field.  Observe  now  whether  any  lines 
can  be  seen  upon  the  surface  of  the  diatoms.  If 
not,  vary  the  distance  of  the  mirror  from  the 
object,  if  adjustment  is  provided  for;  or,  if  lamp 
light  is  used  bring  the  lamp  closer  to,  or  remove 
it  from  the  instrument  in  one  line,  so  that  the 
illumination  will  not  disappear.  If  this  does  not 
bring  out  the  lines,  swing  the  mirror-bar  from  the 
central  position  into  an  oblique  one,  on  the  side 
opposite  to  that  of  the  light  and  readjust  the 
mirror.  Grasp  the  ends  of  the  mirror-fork  be- 
tween the  thumb  and  middle  finger  and  move  the 
mirror  by  the  first  finger.  If  the  field  cannot  be 
evenly  illuminated,  it  is  evident  that  the  mirror  is 
beyond  the  limit  of  angular  aperture  of  the  objec- 
tive and  it  must  therefore  be  brought  back  until 
the  light  appears  equally  well  over  all  parts  of  the 
field.  It  must  here  also  be  noticed  that  if  the 
diaphragm  is  still  attached  to  the  instrument  and 
does  not  swing  with  the  mirror,  it  may  also  be  the 
means  of  cutting  off  light.  The  largest  opening 
should  be  used  or  in  the  case  of  the  cap  dia- 


139 

phragm,  remove  it.  By  means  of  the  micrometer 
screw  carry  the  fine  adjustment  back  and  forth 
beyond  the  plane  of  the  object  and  observe  closely 
whether  any  lines  can  be  distinguished.  It  is  very 
probable  that  they  .will  show ;  but  if  not,  the 
cause  should  be  determined.  It  may  be  that  the 
magnifying  power  is  not  sufficiently  great  and  in 
this  case  a  higher  power  eyepiece  should  be  used, 
or  the  cover  glass  may  be  more  or  less  than  the 
normal  thickness,  which  would  cause  a  spherical 
over  or  under-correction  in  the  objective.  In  this 
case  the  lines  would  appear  when  the  diatom  is 
not  in  focus.  If  the  objective  is  a  non-adjustable 
one,  the  proper  correction  may  be  approximately 
reached  by  means  of  the  draw-tube.  If  the  lines 
appear  over  the  plane  of  the  object,  it  shows  over- 
correction  and  the  length  of  tube  should  then  be 
decreased,  or  contrary  when  the  lines  show  below 
or  beyond  the  plane  of  the  object.  If  the  above 
directions  have  been  followed,  the  lines  cannot 
fail  to  be  seen  with  a  moderately  good  J-  or  \  inch 
objective  ;  but  if  they  are  not,  the  trial  should  be 
repeated.  Again,  be  careful  to  have  no  obstruc- 
tion between  the  course  of  rays  from  the  mirror 
to  the  stage  ;  get  good  illumination  on  the  object ; 
observe  well ;  keep  the  instrument  in  such  a  posi- 
tion that  the  object  is  not  illuminated  from  any 
other  direction  than  from  the  mirror. 

When  the  diatoms  are  resolved  in  this  manner 
the  lines  will  appear  to  be  diagonal  in  some,  longi- 


140 

tudinal  or  transverse  in  others,  according  to  their 
position  and  if  the  resolution  is  very  good,  these 
lines  will  further  resolve  themselves  into  minute 
beads  of  a  hexagonal  form. 

It  will  now  be  well  to  bring  the  mirror  more 
nearly  to  a  central  position.  Do  this  at  intervals 
of  about  10°  and  note  the  appearance  at  each 
decrease  of  obliquity.  It  will  be  found  that  as  the 
mirror  approaches  the  optical  axis  the  lines  will 
appear  to  become  more  faint  and  may  disappear 
before  central  illumination  is  reached  ;  in  this 
case  it  will  be  well  to  begin  again.  An  endeavor 
should  be  made  to  make  each  attempt  give  better 
results  than  the  preceding  one.  Repeated  trials 
will  not  only  impress  the  various  phenomena  upon 
the  mind,  but  will  cause  a  notable  improvement  in 
manipulative  skill  and  thus  a  better  performance 
in  the  objective. 

To  Judge  Spherical  Aberration. — This  is  a 
matter  of  experience  based  upon  a  knowledge  of 
the  principles  involved  and  after  having  been 
studied,  will  be  found  to  be  of  the  utmost  value  in 
utilizing  the  capacity  of  a  microscope.  To  judge 
spherical  aberration  by  the  use  of  histological  or 
biological  objects  without  a  previous  knowledge, 
acquired  from  objects  which  are  more  suited,  is 
extremely  difficult.  One  may  be  aware,  by  the 
unsatisfactory  appearance  , of  the  image,  that 


141 

something  is  amiss,  but  will  probably  not  know 
how  to  correct  the  deficiency. 

Using-  an  objective  J,  •£•  or  -J-  inch  and  the  test 
object  P.  angulatum,  select  a  diatom  which  is  flat 
and  move  to  the  center  of  the  field.  Focus  care- 
fully so  that  the  margin  of  the  object  will  be 
sharply  defined  and  observe  the  markings.  If 
they  show  in  the  same  plane  without  any  further 
focusing-,  the  spherical  correction  may  be  taken  as 
being  correct.  If,  however,  the  lines  appear  to  lie 
in  a  higher  plane  and  it  is  necessary  to  focus 
upward  so  that  the  margin  of  the  diatom  is  out  of 
focus,  it  indicates  spherical  over-correction  and 
the  remedy  is  found  in  the  contraction  of  the  tube 
length.  This  should  be  done  progressively  in 
spaces  of  about  \  inch  and  after  each  change,  care- 
fully focus  again  until  proper  correction  is 
obtained. 

When  the  lines  appear  to  lie  below  the  plane  of 
the  object,  it  indicates  spherical  under-correction 
and  can  be  corrected  by  increasing  the  tube  length. 
If  there  are  two  or  more  eyepieces,  results  can  be 
obtained  quicker  with  the  higher  powers. 

If  the  markings  cannot  be  seen,  it  may  be  due  to 
abnormally  thick  or  thin  covers,  a  not  uncommon 
occurrence,  thus  destroying  the  resolving  power. 
This  may  be  judged  by  using  slight  oblique  illu- 
mination. If  too  much  is  used  the  nice  differences 
will  be  lost. 


Chromatic  Aberration. — This  may  be  judged 
as  described  under  "  Chromatic  Aberration  "  in  a 
a  previous  chapter. 

Cover  Glass. — We  have  thus  far  not  considered 
the  cover  glass,  except  to  show  its  influence  on  the 
optical  performance  of  objectives.  In  preliminary 
examinations  of  solid  objects  with  low  powers  it 
may  be  dispensed  with  ;  but  where  fluids  are  used, 
whether  with  low,  medium,  or  high  powers  it 
should  always  be  used.  A  drop  or  small  quantity 
of  fluid  placed  upon  a  slide  assumes  a  spherical 
form  and,  on  viewing  it  with  a  low  power,  it  will 
be  found  to  give  a  distorted  field  and  will  cause 
disagreeable  reflections  and  shadows. 

In  medium  and  high  powers,  the  front  lenses 
will  be  so  close  to  the  water,  urine,  blood,  etc.,  that 
capillary  attraction  will  often  cause  an  adherence 
to  the  front  surface  of  the  objective  ;  besides  this, 
there  is  such  a  considerable  depth  to  the  fluid  that 
it  obstructs  the  .light,  requires  a  great  change  in 
adjustment  for  the  various  planes  and  is  usually  in 
such  vibration  that  a  sharp  focus  becomes  impos- 
sible. By  merely  dropping  a  cover  glass  upon  it 
all  these  objections  are  overcome. 

The  covers  are  commercially  classified  as  No.  1, 
No.  2  and  No.  3,  but  there  is  a  variation  within  the 


143 

limits   of    different    numbers.     The    variation   is 
about  as  follows  : 

No.  1,  Y^Q  to  -g-^Q  inch,  or  O.i6  to  0.13  mm.  thick  ; 
No.  2,  yfoj.  to  yfo  inch,  or  0.25  to  0.16  mm.  thick  ; 
No.  3  -L.  to  Jf-  inch,  or  0.50  to  0.26  mm.  thick. 


According  to  the  prices  of  cover  glasses,  when 
purchased  by  weight,  the  No.  1  gives  the  greatest 
number  and  No.  3  the  least.  It  may  for  this 
reason  be  thought  that  the  purchase  of  No.  1  is 
most  advantageous,  but  it  must  be  considered  that 
there  is  a  greater  amount  of  breakage  by  cleaning, 
as  they  are  very  thin  and  sensitive.  Considered 
from  the  optical  standpoint,  the  thinner  of  No.  2 
and  the  thicker  of  No.  1  should  be  used,  as  these 
come  within  limits  which  are  adopted  by  opticians 
as  standard  and  to  which  medium  and  high  power 
objectives  are  corrected.  Test  objects  which  are 
prepared  to  test  the  reserving  power  of  objectives 
and  consist  of  diatoms,  are  generally  covered  with 
these  thicknesses. 

An  excellent  means  of  determining  the  thick- 
ness of  cover  glass,  as  well  as  of  studying  spherical 
and  chromatic  aberration,  is  the  Abbe  test-plate. 
This  consists  of  a  series  of  cover  glasses  ranging 
in  thickness  from  0.09  mm.  to  0.24  mm.,  silvered 
on  the  under  surface,  with  lines  cut  through  the 
film,  and  cemented  to  a  slide,  each  cover  being 
marked  with  its  thickness. 


144 

Spherical  aberration  is  corrected  when  the  bands 
or  lines  show  distinctly  without  any  nebulous 
fringe,  and  thus  indicate  the  proper  thickness  of 
cover  to  use. 

Chromatic  correction  may  be  judged  by  the 
character  of  color  bands  which  show  with  oblique 
light.  If  the  bands  are  narrow  and  the  compli- 
mentary colors  of  the  secondary  spectrum,  yellow- 
green  to  apple-green  on  one  side  and  violet  on  the 
other,  indicates  the  best  correction.  For  deter- 
mining the  thickness  of  cover  glass  and  best  cor- 
rection for  spherical  aberration  we  refer  to  the 
cover  glass  gauge  designed  by  the  writer. 

If  the  cover  glass  is  first  measured  with  this 
before  the  object  is  mounted,  the  tube  length  is 
given  for  proper  correction  for  each  thickness  and 
can  be  read  off  on  the  drum.  The  scale  provides 
for  J,  ^,  -J,  and,  ^  inch  objectives  of  the  long  tube 
length  and  for  the  £  inch  of  the  short  tube  length. 
It  also  gives  the  thickness  of  cover  in  millimeters 
and  inches,  and  thus  indicates  the  true  position  of 
adjustment-collar  in  adjustable  objectives. 

Dry,  Adjustable  Objectives. — Adjustable  ob- 
jectives, or  objectives  with  collar  correction  are 
those  in  which  there  is  a  provision  to  vary  the  dis- 
tance between  lenses,  in  order  to  make  proper  cor- 
rection for  spherical  aberration  with  facility.  They 
involve  a  high  degree  of  mechanical  perfection  and 
are  therefore  more  expensive  than  objectives  with 


145 

fixed  mountings.  They  may,  however,  be  recom- 
mended to  microscopists  who  have  acquired  some 
experience  in  handling  objectives  and  even  to  be- 
ginners who  will  use  judgment  in  their  use,  as 
they  certainly  give  excellent  results  and  quick 
means  for  obtaining  the  utmost  limit  of  efficiency 
in  objectives,  a  fact  best  appreciated  by  those 
who  use  them.  In  the  objectives  as  at  present  con- 
structed by  the  best  makers,  a  milled  collar  is  pro- 
vided, which  in  turning  imparts  a  rectilinear 
motion  to  an  interior  tube,  carrying  the  posterior 
systems  of  the  objectives,  thus  varying  the  dis- 
tances between  them  and  the  front  system,  which 
remains  stationary.  The  screw  collar  is  graduated 
in  such  a  manner  that  the  figures  indicate  the  cor- 
rect point  for  the  proper  thickness  of  cover,  thus 
10  indicating  proper  correction  for  a  cover  of 
0.10  mm.,  16  for  0.16  mm.  and  so  on.  When  set  at 
the  highest  figure,  for  thick  covers,  the  lenses  are 
in  the  closest  position  and  the  adjustment  is  said 
to  be  closed.  When  for  the  thin  covers,  are  farthest 
apart  and  is  then  open. 

In  objectives  of  older  construction  and  in  some 
produced  at  the  present  day,  the  figures  are  arbi- 
trary and  serve  no  other  purpose  than  an  index  for 
reference. 

Close  the  adjustment  before  attaching  the  objec- 
tive as  its  front  may  otherwise  come  in  contact 
with  the  cover  before  the  focus  is  reached.  For 


OF  TH-R 


146 

practice  with  this  objective  use  P.  angulatum. 
Focus  carefully  and  observe  whether  any  lines  can 
be  seen  ;  if  not,  grasp  the  milled  edge  of  the  adjust- 
ment collar  between  the  thumb  and  first  finger  of 
the  left  hand,  keeping  the  fingers  of  the  right  hand 
upon  the  micrometer  screw.  Turn  the  collar 
slightly  toward  its  open  point  and  as  this  will  place 
the  object  out  of  focus,  move  the  fine  adjustment 
correspondingly.  Continue  to  turn  the  collar  little 
by  little  and  do  not  cease  to  observe  closely  ;  also, 
after  each  movement,  focus  above  or  below  the 
plane  of  the  object,  so  that  this  will  be  distinct, 
and  look  for  the  lines.  Possibly  after  a  little  they 
will  begin  to  appear  faintly  ;  but,  if  not,  continue 
to  bring  the  collar  toward  the  middle  point.  The 
lines  must  now  soon  make  their  appearance  and 
when  they  do,  it  will  probably  be  above  the  plane  of 
the  diatom.  This  is  an  indication  that  the  objec- 
tive is  approaching  its  correction  for  the  cover. 
Now  keep  the  lines  in  focus,  while  the  correction 
collar  is  being  gradually  turned,  until  the  lines 
and  the  outline  of  the  diatom  lie  in  one  plane. 
The  objective  is  now  said  to  be  corrected  for  cover. 
Observe  which  number  corresponds  to  the  index, 
and  again  return  the  collar  to  its  closed  point  and 
go  through  the  same  proceedings  as  carefully  as 
at  first.  When  the  best  point  is  again  reached 
look  for  the  number  and  see  whether  it  agrees 
with  the  first  ;  very  likely  it  does  not,  -which  is 


Itt 

owing  to  a  want  in  the  faculty  of  perception,  due 
to  a  too  slight  acquaintance  with  the  phenomena. 
These  trials  should  be  repeated  until  the  proper 
sensitiveness  of  feeling  in  making  the  adjustments 
is  acquired  and  until  they  can  be  made  to  corres- 
pond with  a  certainty  to  at  least  within  two 
divisions.  When  it  is  found  after  repeated  trials 
that  sufficient  skill  has  been  acquired,  mark  the 
number  upon  the  slide.  For  future  examination  of 
the  same  slide,  this  will  facilitate  work  and  give 
the  assurance  that  the  best  results  are  thus  ob- 
tained without  further  trial  by  simply  referring  to 
the  recorded  number. 

On  stained  Bacteria  and  Micrococci,  focus  briskly 
with  the  fine  adjustment  to  either  side  of  exact 
focus.  There  will  be  an  expansion  of  the  outline 
of  the  object  both,  when  within  and  without  the 
focus.  If  the  greater  expansion  or  coma  is  within 
the  focus,  or  when  it  is  necessary  to  raise  the 
objective,  there  is  spherical  over-correction  and 
the  adjustment  must  be  closed.  When  the  proper 
point  of  correction  is  reached  the  expansion  of 
outline  is  the  same  in  both  directions. 

Immersion  Objectives. — As  has  been  stated 
before,  immersion  contact  between  the  objective 
and  cover  glass  is  made  by  either  water  or  homo- 
geneous fluid.  Distilled  water  only  should  be 
used  and  kept  in  a  suitable  bottle.  Cedar  oil  is 
used  for  the  homogeneous  fluid  and  is  thickened 


148 

for  convenience  in  use.  A  small  bottle  is  gener- 
ally supplied  with  each  oil  immersion  objective. 
Grent  care  should  be  used  in  keeping  it  free  from 
dust,  as  it  often  happens  that  an  objective  fails  to 
give  satisfaction,  due  to  a  small  particle  of  dust 
which  may  float  in  the  fluid  before  the  front  lens. 
Great  care  should  also  be  exercised  in  applying  oil 
to  the  front  lens  and  after  the  application,  it  is 
strongly  recommended  to  examine  it  with  a  mag- 
nifier, that  there  may  be  no  air  bubbles  present. 
These  as  well  as  dust  may  seriously  interfere  with 
obtaining  satisfactory  results.  If  bubbles  are 
present  the  oil  should  be  removed  and  a  fresh 
quantity  applied.  Care  should  also  be  taken  not  to 
apply  too  great  a  quantity.  After  the  stopper  has 
been  withdrawn  from  the  oil,  allow  the  oil  to  run 
down  the  rod  or  brush  until  the  last  natural  drop 
has  separated  from  it  and  apply  the  remainder,  or 
less  than  a  drop,  to  the  front  of  the  objective. 

Attach  the  objective  and  lower  it  until  the  fluid 
comes  in  contact  with  the  cover  and  observe  this 
by  lowering  the  head.  Focus  as  with  dry  objec- 
tives. The  use  of  immersion  fluid  in  itself  involves 
a  certain  amount  of  inconvenience,  but  the  obser- 
vance of  fixed  rules  will  materially  help  to  over- 
come some  of  the  disagreeable  features.  Extreme 
cleanliness  should  be  observed  with  it.  After  the 
work  has  been  completed  the  objective  should  be 
removed  from  the  stand  and  its  front  as  well  as 


149 

the  slide  should  invariably  be  cleaned.  The  fluid 
may  be  removed  by  a  moist  piece  of  soft  linen  and 
the  front  then  cleaned  with  a  dry  piece  or  with 
lens  paper.  Chamois  skin  is  not  suitable,  as  it  does 
not  absorb  the  fluid. 

Immersion  Objectives  on  Test  Plate. — From 
the  fact  that  in  oil  immersion  objectives  the  fluid 
has  the  same  refractive  index  as  the  cover  and 
front  of  the  lens,  it  is  not  sensitive  to  variations  in 
thickness  of  cover,  although  many  of  the  most 
expert  manipulators  prefer  adjustable  mountings 
in  order  to  obtain  the  highest  results,  if  any  dis- 
turbing element  should  be  present. 

To  determine  the  highest  capacity  on  test  objects, 
ordinary  daylight  is  not  sufficient ;  a  flat  wick  oil 
lamp  is  best  suited.  If  the  right  hand  is  used  on 
micrometer  screw,  place  the  lamp  at  the  right  side 
of  the  instrument,  about  ten  inches  from  it,  with 
the  edge  of  the  flame  turned  toward  the  mirror. 

Place  the  test  plate  upon  the  stage  and  as  the 
diatoms  in  balsam  are  very  transparent  and  there- 
fore very  difficult  to  find,  a  lower  power  objective 
may  be  used  as  a  finder  ;  bring  No.  1,  or  Trice- 
ratium  favus  into  the  center  of  the  field  and  after 
the  objective  has  been  removed,  attach  the  immer- 
sion objective,  which  we  assume  to  be  a  y^,  in  the 
manner  prescribed.  Get  the  best  possible  illumi- 
nation with  the  mirror  at  the  central  point  and 
move  the  test  plate  from  diatom  to  diatom  until  it 


150 

reaches  No.  11,  P.  angulatum,  but  observe  closely 
the  structure  of  each  one  as  it  comes  into  the  field. 
Next,  see  whether  the  objective  is  corrected.  If 
the  lines  and  outlines,  or  middle  rib,  do  not  appear 
to  be  in  one  plane,  adjust  the  collar  in  adjustable 
and  the  tube  length  in  non-adjustable  objectives 
until  they  are  and  then  continue  the  advance 
toward  the  higher  numbers  until  one  is  reached  on 
which  no  lines  can  be  seen.  Swing  the  mirror-bar 
to  an  obliquity  of  20°  to  the  left  side  and  readjust- 
ing the  mirror,  observe  the  effect.  It  is  very 
probable  that  the  lines  will  show  and  if  so,  con- 
tinue the  advance  ;  if  they  do  not,  give  10°  or  20° 
more  obliquity  and  after  the  structure  comes  out, 
again  go  forward.  A  point  may  thus  be  reached, 
where  with  the  greatest  obliquity  which  can  be  given 
and  with  the  best  possible  illumination,  the  objec- 
tive seems  to  have  come  to  the  limit  of  its  perfor- 
mance. From  the  claims  which  have  been  made 
for  it,  it  ought  to  do  better.  What  is  the  cause 
of  failure  ?  Possibly  the  mirror  is  not  correctly 
focused,  or  the  adjustment  collar  may  not  be  cor- 
rect for  oblique  light ;  perhaps  the  eyepiece  does 
not  give  sufficient  magnifying  power  to  distinguish 
the  stricz.  It  may  be  any  one  of  these  causes  or  all 
combined.  As  to  the  eyepiece,  the  manipulator 
must  remember  the  amount  of  separation  of  lines 
in  the  last  object  which  was  resolved  and  from  the 
gradation  in  the  coarser  specimens  must  judge 


whether  the  power  is  sufficient  ;  it  should  be  added 
that  for  any  over  No.  14  and  under  No.  18  a  1  inch 
eyepiece  should  be  used  and  for  those  above  No. 
18  a  power  of  f  inch  will  probably  be  necessary. 
After  this  condition  has  been  complied  with,  look 
to  the  correction  collar  of  the  objective.  To  obtain 
the  highest  results  it  very  often  occurs  that  a  dif- 
ferent adjustment  is  required  for  obliqtie  light 
from  that  for  central  light.  Note  the  number  at 
which  the  collar  stands  and  then  work  it  back  and 
forth,  watching  carefully  for  results.  If  this  has 
no  influence,  return  it  to  its  number  or  to  a  point 
where  the  outline  of  the  object  appears  most  sharp. 
Now  look  to  the  illlumination  ;  vary  the  distance 
of  the  mirror  to  the  object,  or  if  this  cannot  be 
done,  vary  the  distance  of  the  lamp  to  the  instru- 
ment and  watch  the  effect  of  the  change  through 
the  eyepiece.  If  neither  of  these  changes  give  any 
improvement,  recourse  must  be  had  to  another 
expedient.  Place  a  bull's-eye  between  the  lamp  and 
mirror  with  the  plane  side  of  the  lens  toward  the 
lamp  and  close  to  it,  so  that  the  light  is  thrown 
on  the  mirror.  It  should  be  properly  concen- 
trated, so  that  the  circle  of  light  will  not  be  larger 
than  the  mirror,  which  can  be  determined  by  plac- 
ing the  hand  or  a  piece  of  paper  back  of  it.  Adjust 
when  necessary  by  moving  the  lamp  or  bull's-eye. 
Keep  it  a  little  below  the  line  of  the  top  of  the 
stage,  so  that  the  beam  from  the  bull's-eye  will  not 


152 

strike  it  on  its  upper  surface  and  as  little  as  pos- 
sible on  its  lower  surface.  If  the  direct  light  from 
the  bull's-eye  reaches  the  object,  it  destroys  to 
some  extent  the  effect  of  the  oblique  illumination 
from  the  mirror.  Great  care  should  be  given  to 
this  point,  as  it  is  very  important. 

If  all  of  these  suggestions  have  been  followed,  a 
great  difference  will  undoubtedly  be  noticed  in  the 
performance  of  the  objective  ;  but  if  it  still  does 
not  come  up  to  the  standard,  patience  must  not  be 
lost.  The  slightest  change  in  the  position  of  the 
mirror,  bull's-eye,  or  lamp,  or  a  touch  to  the  cor- 
rection collar  or  micrometer  screw,  is  sometimes 
followed  by  astonishing  results.  The  beginner 
should  sit  down  with  the  expectation  that  he  will 
fail  at  the  first  trial.  At  each  succeeding  trial  he 
can  easily  notice  his  improvement  in  manipula- 
tion and  a  corresponding  gain  in  the  results.  He 
should  be  able  to  bring  the  performance  of  the 
objective  up  to  the  claims  made  for  it,  if  it  has 
come  from  the  hands  of  a  reliable  optician  and 
should  not  rest  until  this  is  accomplished. 

The  writer  has  often  recommended  sunlight 
with  generally  successful  results  where  ordinary 
means  of  illumination  have  failed.  The  light  is  of 
course  intense  and  great  care  will  have  to  be  used 
to  modify  it  by  properly  using  the  mirror,  .but  suc- 
cess is  often  attained  and  then  creates  confidence. 


153 

It  is,  however,  only  recommended  for  this  purpose 
and  not  for  general  use. 

Stained  Bacteria  and  Micrococci  also  make 
excellent  objects  for  immersion  objectives.  The 
mode  of  illumination  is  the  same  as  with  dry 
objectives. 


ILLUMINATION  WITH  CONDENSER. 


Up  to  this  point  the  matter  of  illumination  has 
been  treated  in  its  most  simple  form  with  mirror 
only,  as  most  instruments  are  of  the  simple  type, 
but  we  must  now  consider  the  substage  condenser, 
which  is  a  most  valuable  adjunct  to  the  micro- 
scope. While  skillful  treatment  of  the  microscope 
will  go  far  toward  obtaining  good  results,  it  will  in 
many  investigations  not  suffice.  Furthermore  the 
condenser  will  give  results  with  ease  which'  without 
it  involve  effort  and  skill. 

Purpose  of  the  Condenser. — The  purpose  of 
the  condenser  is  not  only  as  its  name  implies,  to 
condense  light,  but  is  more  especially  to  illuminate 
the  object  with  a  cone  of  light  having  an  angular 
aperture  equal  to  that  of  the  objective  used  and 
which  is  absolutely  unattainable  with  a  mirror 
only,  as  well  as  to  provide  means  for  controlling 
the  amount  and  character  of  the  illumination  to 
suit  the  various  conditions  of  work. 

Abbe  Condenser. — The  history  of  substage 
condensers  is  very  unique  and  interesting  and 
shows,  how  from  having  been  the  subject  of  no 
end  of  condemnation,  which  for  many  years  it 


155 

received,  it  is  now  generally  accepted  as  a  neces- 
sary adjunct  to  a  complete  outfit,-  in  fact  should 
always  accompany  an  oil  immersion  objective. 
From  single  lenses,  compound,  non-achromatic 
and  achromatic,  the  use  of  eyepieces  and  objec- 
tives as  condensers  with  any  number  of  devices 
for  regulating  the  light,  the  generally  accepted 


Fig.  39. 

Optical  part  of  condenser  1.20  Aperture. 


Fig.  40. 
Optical  part  of  condenser  1.42  Aperture. 

forms  have  come  to  be  those  devised  by  Prof. 
Abbe.  One  of  them  with  a  numerical  aperture  of 
1.20  consists  of  a  combination  of  two  lenses  (Fig. 
39)  and  the  other  with,  an  aperture  of  1.42  of  three 
lenses  (Fig.  40).  A  third  is  made  achromatic  with 
an  aperture  of  1.0  which,  however,  is  considerably 
more  expensive. 


156 

The  most  simple  form,  largely  used  for  instru- 
ments for  laboratory  and  everyday  work  is  one 
which  has  attached  to  it  below  an  iris  diaphragm 
for  regulating  the  amount  and  angle  of  light  and 
to  which  is  attached  a  swinging  arm  to  receive 


Fig.  41. 


15? 

blue  glass  for  moderating"  light,  or  stops  for  dark 
ground  or  oblique  illumination.  A  screw  motion 
gives  a  serviceable  means  of  adjustment. 

The  most  complete  form  is  that  shown  in  Fig.  41, 
which  has  adjustments  for  obtaining  every  modifi- 
cation and  character  of  illumination,  with  rack  and 
pinion  for  vertical  adjustment  and  swinging  out  of 
the  way  the  condenser  and  iris  diaphragm  if  it  is 
not  desired  to  use  these. 

A  condenser  while  useful  may  be  abused  so  as  to 
do  more  harm  than  good  and  we  deem  it  proper 
to  give  some  instructions  in  its  use,  which  we  trust 
may  be  of  service. 

Use  only  plane  mirror  with  the  condenser. 

Centering  the  Condenser. — Every  condenser 
should  have  a  centering  arrangement,  so  as  to 
bring  its  axis  coincident  with  that  of  the  objective. 
In  the  simple  form  of  microscopes  in  which  the 
character  of  work  is  not  critical,  the  condenser  is 
sufficiently  centered  for  all  ordinary  requirements, 
but  even  here  it  should  be  possible  to  center  if 
conditions  should  demand  it. 

In  the  more  complete  apparatus  a  pin  hole  cap 
to  aid  in  centering  the  condenser  and  fitting  over 
the  upper  part  of  the  mounting  should  be  supplied, 
and  after  focusing  upon  the  opening  in  the  cap 
with  the  low  power  objective,  bring  into  the  center 
of  field  with  the  centering  screws.  Closer  adjust- 


158 

ment  can  be  made  after  the  high  power  objective 
is  attached. 

In  the  simple  form,  pin  hole  cap  is  usually  not 
provided.  To  verify  correct  centering-  two  easy 
methods  may  be  followed. 

1.  Use  a    2  inch  objective  and  focus  through  the 
condenser  onto  the  diaphragm,  which  is  reduced  to  its 
smallest  opening. 

2.  Use  a  f  or  f-  inch  objective  and  bring  to  its  focal 
point ;  remove  eyepiece  and  look  through  the   tube, 
keeping  the  eye  in  center,  when  the  small  opening  of 
diaphragm  will  be  seen  sharply  defined. 

This  opening  in  diaphragm  may  be  enlarged  to 
the  full  aperture  of  objective  so  that  the  margin 
of  opening  in  the  diaphragm  will  just  coincide 
with  that  of  the  back  lens  of  objective. 

Centering  the  Illumination. — This  is  fully  as 
important  as  centering  the  condenser,  tor  it  is  of 
little  avail  to  have  a  centered  condenser  when  the 
projected  beam  of  light  is  not  centered.  This 
depends  upon  the  source  of  light  and  position  of 
mirror  and  when  it  is  remembered  that  the  mirror 
may  be  so  adjusted  as  to  give  all  gradations  of 
oblique  illumination  from  the  central  to  the  limit 
of  aperture,  the  necessity  for  accurate  adjustment 
becomes  apparent. 

Attach  f  or  f  inch  objective ;  open  diaphragm  to 
full  extent  and  focus  upon  the  minute  image  of 


159 

flame  ;  adjust  mirror  so  that  the  image  will  be  in 
the  center  of  field. 

To  Focus  Condenser. — Since  the  condenser  is 
nothing  more  than  a  combination  of  lenses  similar 
to  those  in  an  objective,  but  used  in  a  reversed 
position,  it  has  the  same  properties  of  angular 
aperture,  focus  and  working  distance.  But,  as  it 
must  work  through  the  thickness  of  slide,  its 
working  distance  is  proportionately  large  to  the 
angular  aperture  and  owing  to  the  variation  in 
thickness  of  slides,  its  focus  is  variable,  falling  in 
some  slides  above  and  in  others  below  its  effective 
position.  In  the  simple  mountings  the  condenser 
is  generally  so  placed  that  its  upper  surface  is  just 
below  the  surface  of  the  stage  and  for  the  low  and 
medium  powers  this  will  be  generally  found  suf- 
ficiently close,  in  fact  at  this  point  it  may  often  be 
found  that  the  illumination  is  too  intense  and  may 
then  be  reduced  either  by  reducing  the  aperture 
of  the  diaphragm,  or  by  removing  the  condenser 
farther  from  the  object. 

In  all  objectives  with  a  numerical  aperture  less 
than  1.0  the  condenser  may  be  used  dry,  without  oil. 
In  immersion  objectives  the  top  of  condenser 
should  be  brought  in  fluid  contact  with  the  slide. 
The  same  phenomena  take  place  by  refraction  of 
the  light  from  condenser  through  the  slide,  as 
from  an  object  through  the  cover  glass  to  the 
objective.  To  make  contact,  place  a  drop  of  oil 


160 

on  the  top  of  condenser,  drop  the  slide  upon  the 
stage,  first  throwing  the  clips  to  one  side.  With 
immersion  objectives  the  proper  focusing  of  the 
condenser  becomes  a  matter  of  nice  distinction  to 
obtain  best  results  and  can  only  be  reliably  accom- 
plished by  considerable  practice  and  experience. 

To  obtain  best  position  use  a  f  objective  ;  focus  upon 
the  object,  after  the  slide  has  been  brought  in  fluid 
contact  with  condenser  and  tJien  adjust  condenser 
until  image  of  flame  will  fall  in  the  same  plane  with 
the  object, 

Relation  of  Aperture  of  Condenser  to  Ob- 
jective.— As  has  been  stated  the  condenser  may 
be  the  means  of  doing  more  harm  than  good, 
depending  mainly  upon  the  angle  of  illumination 
used.  Too  much  illumination  decidedly  injures 
definition  by  obliterating  detail.  In  the  study  of 
Bacteria  and  Micrococci,  with  which  the  objectives 
used  are  of  wide  aperture,  it  is  sought  to  have 
them  stand  out  boldly  in  a  bright  field,  which  is 
accomplished  by  bringing  the  diaphragm  to  its 
full  aperture. 

In  all  dry  objectives  the  aperture  of  the  condenser 
should  be  less  than  that  of  the  objective. 

Little  experience  is  required  to  judge  when  the 
condenser  has  its  proper  opening.  When  correct, 
the  image  will  stand  out  sharply  defined  without 
any  appearance  of  fogginess  and  as  the  aperture  is 


101 

reduced,  it  will  be  noticeable  by  the  decrease  in 
the  amount  of  light.  By  removing  the  eyepiece 
and  looking  at  the  back  of  the  objective  the  rela- 
tive aperture  of  the  condenser  to  that  of  the  objec- 
tive may  be  easily  seen,  as  the  outlines  of  the 
diaphragm  are  sharply  defined.  If  the  opening 
in  diaphragm  appears  to  have  the  same  opening 
as  the  back  of  objective,  the  condenser  has  the 
same  angular  aperture.  By  experience  the  fol- 
lowing conditions  have  been  found  to  give  most 
satisfactory  results  : 

In  oil  immersion  objectives  on  bacteria  use  the 
full  aperture  of  condenser. 

On  diatoms  reduce  the  aperture  to  about  two- 
thirds  opening  in  objective. 

In  histological  and  other  dense  objects  use  an 
aperture  equal  to  about  one-half  the  opening  of  back 
lens  in  objective. 

Oblique    Light    With    Condenser.— Oblique 

light  may  be  obtained  by  setting  the  mirror  in 
such  a  position  that  the  light  reflected  from  it 
shall  enter  the  condenser  only  at  one  side,  leaving 
the  balance  of  it  unused.  This,  however,  is  only 
advisable  when  the  condenser  mounting  has  no 
other  provision  for  obtaining  oblique  light.  In 
the  mountings  however,  having  such  provision,  it 
is  obtained  by  reducing  the  opening  in  diaphragm 
so  that  it  shall  correspond  in  size  to  the  back 


162 

lens  of  objective  and  then  moving  this  opening 
laterally  across  the  bottom  of  condenser  to  such  a 
point  as  will  bring  out  the  structure  of  the  object, 
or  to  the  limit  of  the  aperture  of  the  objective. 
When  beyond  the  limit  of  aperture  the  field  will 
appear  to  become  dark.  The  amount  of  illumina- 
tion may  be  modified,  but  in  a  general  way  it  may 
be  said  that  the  best  results  with  oblique  illumina- 
tion are  obtained  by  reducing  the  amount  of 
illumination  to  its  minimum  practicable  amount. 
In  objects  with  striated  structure,  the  illuminat- 
ing rays  should  be  brought  to  a  position  at  right 
angles  to  the  striae,  either  by  rotating  the  object  to 
the  proper  position,  or  by  swinging  the  diaphragm. 


HOW  TO   DRAW   OBJECTS. 


To  be  able  to  make  a  correct  drawing  of  the 
enlarged  image  of  an  object  is  very  important  and 
while  for  some  lines  of  work  the  photographic 
camera  is  called  into  requisition,  drawings  are 
nevertheless,  largely  relied  upon.  The  apparatus 
requisite  for  this  purpose  is  the  camera  lucida  and 
while  there  is  a  variety  of  forms,  all  are  based 
upon  the  principle  of  causing  the  image  of  the 
object  to  appear  projected  upon  the  paper,  where 
it  may  be  drawn. 

The  most  simple  form  is  that  shown  in  Fig.  42. 


Fig.  42. 

With  this  the  microscope  must  be  considerably 
inclined  and  the  camera  lucida  attached  to  the  eye- 
piece. The  emergent  pencil  is  reflected  by  the 


164 

thin  film    of   glass   into  the   eye    and  apparently 
projected  through  the  glass  upon  the  paper. 

Another  simple  form  is  the  Wo  lias  ton  camera 
lucida  (Fig.  43),  which  is  attached  to  the  eyepiece 
in  the  same  manner  as  the  foregoing,  with  the 


^  £3- 


Fig.  43. 

instrument  also  inclined.  This  consists  of  a  quad- 
rangular prism,  which  reflects  the  emergent  pencil 
through  a  small  opening  into  the  eye.  The  eye  is 
placed  over  the  edge  of  the  prism  in  such  a  man- 
ner that  the  rays  forming  the  image  enter  only  a 
portion  of  the  pupil,  while  its  other  portion  views 
the  paper.  In  using  this  however,  the  eye  must 
be  kept  quite  steadily  in  the  proper  position. 

While  there  are  some  other  forms,  they  are  not 
in  general  use.  The  very  best  is  one  designed  by 
Prof.  Abbe,  and  which  goes  under  the  name  of  the 
Abbe  camera  lucida  (Fig.  44).  It  is  fixed  to  the 
tube  of  the  microscope,  permitting  the  mounting 
which  contains  the  revolving  prism  to  be  swung 


165 

back,  so  that  the  eye  may  be  applied  to  the  eye- 
piece. It  is  composed  of  two  rectangular  prisms, 
cemented  together  at  their  diagonal  surfaces,  but 
having  between  them  a  perforated  film  of  silver. 


Fig.  44. 

At  one  side  of  the  prism  is  a  large  mirror  which 
reflects  the  image  of  the  paper  and  pencil  onto  the 
silvered  surface  of  the  cemented  prism  and  this 
in  turn  to  the  marginal  portion  of  the  pupil, 
while  the  axial  portion  of  the  eye  views  the  image 
through  the  perforation  of  the  silvered  surface, 
thus  giving  both  views  coincidentally. 

A  number  of  precautions  are  to  be  taken  to 
obtain  good  results.  First  of  all  it  must  be 
remembered  that  if  spectacles  are  necessary  in 


166 

reading,  these  will  be  required  in  order  to  see  the 
pencil  point  clearly. 

The  pencil  point  should  be  well  sharpened  in 
order  to  closely  follow  the  minute  detail. 

The  drawing  board  should  be  at  right  angles  to 
the  axial  portion  of  the  projected  image,  otherwise 
there  will  be  an  elongated  picture. 

The  relative  illumination  of  field  and  paper 
varies  with  magnifying  power  and  distance  of  eye- 
piece from  paper,  but  should  be  made  as  nearly 
alike  as  possible.  In  low  powers  the  light  from 
the  objective  is  stronger  than  that  from  the  paper 
and  should  be  modified,  in  the  simple  and  Wollas- 
ton  forms  by  using  the  plane  mirror  or  by  cover- 
ing it  with  white  tissue  paper,  and  in  the  Abbe 
form  by  the  use  of  dark  tinted  glass,  which  is  pro- 
vided with  the  apparatus.  In  the  higher  powers 
the  paper  is  brighter  in  which  case  it  should  be 
shaded  by  a  screen  placed  between  it  and  the  source 
of  light,  or  a  substage  condenser  should  be  used 
to  give  increased  illumination. 

Special  instructions  with  reference  to  the  use  of 
the  simple  and  Wollaston  forms  are  as  follows  : 

Focus  the  objective  upon  the  object ;  then  in- 
cline the  microscope,  preferably  to  the  horizontal 
position ;  raise  the  microscope  by  underlaying 
with  a  block  or  books.  The  size  of  drawing  may 
be  varied  by  change  of  eyepiece  or  objective,  but 
only  to  a  limited  extent  ;  or  by  varying  the  dis~ 


107 

tance  between  eyepiece  and  table,  it  being-  appar- 
ent that  as  the  distance  becomes  greater  the 
magnification  will  proportionately  increase.  The 
usual  standard  for  distance  is  10  inches,  although 
this  may  be  varied  to  suit  requirements.  Attach 
the  camera  lucida ;  place  the  paper  upon  the 
table  and  fasten  with  drawing  tacks  ;  apply  the 
eye  to  camera  lucida  ;  refocus  carefully  with  the 
fine  adjustment  and  endeavor  to  obtain  equally 
sharp  view  of  image  and  pencil  point.  In  these 
two  forms  it  is  quite  difficult  to  see  the  pencil 
point  clearly  and  some  care  will  be  required  in 
modifying  the  light  or  shading  the  paper  so  that 
the  image  and  pencil  point  may  be  sufficiently  dis- 
tinct at  the  same  time  to  follow  the  outlines. 

In  the  Abbe  camera  lucida  the  optical  results  are 
considerably  better,  since  a  direct  view  of  the 
image  is  obtained  and  the  equalization  of  illumina- 
tion is  easily  accomplished. 

While  with  it,  the  microscope  is  intended  to  be 
used  in  an  upright  position,  the  reflecting  mirror 
is  close  to  the  eye  so  that,  if  the  image  is  projected 
vertically,  a  portion  of  the  field  is  cut  off  by  the 
stage  or  base  of  the  microscope  and  if  the  mirror 
is  so  inclined  as  to  bring  the  full  field  upon  the 
paper,  it  will  not  be  round,  but  elongated  or  ellipti- 
cal and  thus  also  elongate  the  image.  This  can 
only  be  corrected  by  correspondingly  raising  one 
end  of  the  drawing  board  so  that  it  shall  be  at 


168 

right  angles  to  the  axis  of  projected  cone,  or  until 
the  field  appears  round.  A  convenient  method  of 
accomplishing  this,  with  the  further  advantage  of 
being  able  to  elevate  the  drawing  board  and  thus 
produce  a  variation  in  the  size  of  drawing,  is  the 
drawing  table  made  by  Bausch  &  Lomb  Optical  Co. 
Care  must  also  be  used  in  attaching  the  camera 
lucida  to  the  tube,  so  that  the  opening  in  the 
prism  shall  be  in  the  optical  axis.  This  can  be 
accomplished  by  the  centering  screws  and  observ- 
ing whether  any  of  the  field  is  cut  off.  If  the  field 
appears  smaller  with  the  prism  than  by  directly 
looking  into  the  eyepiece,  the  prism  is  too  close,  or 
too  distant  from  the  eye  lens  and  should  be  prop- 
erly adjusted. 

No  particular  skill  is  required  in  drawing  as  it 
is  simply  a  question  of  copying,  but  the  lines 
should  be  light  so  that  any  irregularities  may  be 
corrected  after  the  work  with  the  camera  lucida  is 
completed.  To  determine  the  standard  distance 
of  10  inches,  measure  from  the  optical  axis  to  the 
axis  of  mirror  and  from  this  point  to  drawing 
paper. 

If  it  is  desired  to  determine  the  amount  of 
enlargement  of  the  image  on  paper,  this  can  be 
done  by  replacing  the  object  by  a  stage  micrometer 
and  drawing  its  spaces  over  the  object.  If  the 
divisions  or  spaces  are  0.01  mm.  and.  it  were  found 
that  10  spaces  covered  the  object  and  the  actual 


169 

measurement  was  80.0  mm.  the  enlargement 
would  be  30  times  100  divided  by  10,  or  300.  If  a 
standard  of  10  inches  is  maintained  in  all  drawings 
and  the  amount  of  magnification  with  certain 
objectives  and  eyepieces  be  previously  determined 
by  means  of  the  stage  micrometer,  a  standard  is 
established  for  each  and  further  measurements 
will  not  be  required.  If  however,  variations  from 
this  standard  distance  are  made,  the  actual  magni- 
fication should  always  be  determined. 

To  Determine  Magnifying  Power. — While 
the  magnifying  power  may  be  known  from  tables 
accompanying  the  microscope,  these  are  only 
approximate,  as  there  is  more  or  less  variation  in 
eyepieces  and  objectives  and  furthermore  the 
microscope  may  be  used  under  different  conditions. 
There  are  three  requisites. 

A  camera  lucid  a. 

A  stage  micrometer  ruled  in  divisions  of  inches  or 
millimeters. 

A  pocket  or  foot  rule  in  inches  or  millimeters, 
according  to  the  stage  micrometer  which  is  used. 

If  a  low  power  objective  is  used,  place  the  stage 
micrometer  with  divisions  of  0.1  mm.  or  0.01  inch 
upon  the  stage  and  focus.  After  attaching  the 
camera  lucida,  place  the  microscope  in  exactly  the 
same  position  as  for  drawing,  maintaining  the 
standard  distance  of  10  inches  from  optical  axis  to 


170 

drawing  paper  ;  mark  the  spaces  of  the  micrometer 
as  projected  upon  the  paper  and  determine  how 
many  of  the  divisions  of  the  rule  are  contained 
within  one  or  more  spaces  on  the  paper.  If  the 
values  are  in  millimeters  and  it  should  be  found 
that  25.0  mm.  on  the  rule  are  contained  in  one 
space  on  the  paper,  the  magnification  would  be 
250.  If  18.6  on  the  rule  are  contained  within 
three  spaces  on  the  paper,  the  magnification  would 
be  63  times. 

To  Measure  the  Size  of  an  Object. — One  of 

the  most  valuable  possibilities  of  the  microscope  is 
to  be  able  to  accurately  measure  the  actual  size  of  a 
minute  object.  Computations  may  be  made  in 
inches  or  millimeters  by  figuring  25.4  mm.  equal 
to  1  inch.  It  may  be  done  by  several  methods, 
two  of  which  are  generally  followed. 

The  first  of  these  give  satisfactory  results  on 
coarser  objects  and  wherever  the  most  accurate 
results  are  not  required,  although  it  is  somewhat 
inconvenient.  The  requisites  are 

A  camera  lucida. 

A  stage  micrometer. 

The  object  is  placed  upon  the  stage  and  after 
focusing,  the  camera  lucida  is  attached  and  the 
instrument  set  up  exactly  as  for  drawing.  On  the 
drawing  paper  the  outlines  of  object,  or  that  por- 
tion which  is  to  be  measured  are  marked,  without 


in  the  least  disturbing  any  of  the  conditions  of 
tube  length  or  distance  from  the  paper.  Remove 
the  object  and  replace  it  by  the  stage  micrometer, 
focusing  only  with  the  fine  adjustment  and,  it  may 
be  added,  there  should  be  very  little  variation  in 
thickness  between  the  two  slides.  Move  the  micro- 
meter so  that  one  of  its  lines  shall  exactly  coincide 
with  one  end  of  the  drawing  on  the  paper  and  then 
measure  off  how  many  spaces  are  covered  by  the 
object.  Thus  if  0.001  inch  are  the  values  of  the 
micrometer  spaces  and  the  object  covers  one  space, 
its  size  will  be  0.001  inch,  or  covering  7  spaces  will 
be  0.007  inch. 

A  variation  of  the  distance  of  the  camera  lucida 
from  the  paper,  or  a  change  of  power  in  eyepiece 
or  objective  does  not  vary  the  result  so  long  as 
objective  and  micrometer  are  used  under  exactly 
the  same  conditions. 

The  second  method  is  with  the  eyepiece  micro- 
meter which  in  its  simple  form  is  a  micrometer 
mounted  on  a  plate  which  is  placed  in  the  focus 
of  the  eye  lens.  It  is  usually  graduated  to  0.10  mm., 
seldom  exceeding  0.05  mm.,  as  closer  lines  cannot 
well  be  distinguished  with  the  magnification 
obtained  by  the  eye  lens.  The  plate  is  dropped 
onto  the  diaphragm,  where  it  is  in  focus,  or  enters 
slots  which  are  provided  in  the  tube  of  the  eye- 
piece. A  better  form  however,  is  the  eyepiece 
micrometer  (Fig.  45)  in  which  the  eyepiece  and 


micrometer  form  a  complete  apparatus  and  a 
lateral  adjustment  of  the  scale  across  the  field  is 
given  by  a  screw. 


Fig.  45. 

In  either  of  these  forms  the  ruled  lines  appear  to 
lie  directly  on  the  image  of  the  object,  but  while 
we  have  on  the  one  hand  the  actual  value  of  the 
micrometer  we  have  on  the  other  only  the  image 
of  the  object.  The  valuation  of  the  eyepiece 
micrometer  in  the  value  of  the  stage  micrometer 
must  be  first  determined.  While  the  optician  can 
do  this,  it  should  be  done  by  the  manipulator  on 
account  of  the  varying  conditions  of  tube  length, 
power  of  objective,  etc.  A  stage  micrometer 
divided  into  the  same  spaces  as  the  eyepiece 
micrometer  is  necessary. 

Focus  the  eye  lens  on  the  eyepiece  micrometer 
and  the  objective  on  the  stage  micrometer,  being 
careful  to  bring  the  first  line  of  the  former  coinci- 


173 

dent  with  a  line  of  the  latter,  using  care  to  see  that 
they  are  parallel.  As  the  lines  of  the  stage  micro- 
meter will  appear  to  have  a  certain  amount  of 
thickness,  make  the  first  line  of  the  eyepiece 
micrometer  correspond  with  one  edge  of  a  line  on 
the  other.  Now  read  off  how  many  of  the  lines 
are  contained  in  one  space  of  the  stage  micrometer 
and  note  this.  We  will  assume  that  it  is  8  divis- 
ions. Replace  the  stage  micrometer  by  the  object 
to  be  measured  and  bring  one  edge  of  the 'object 
coincident  with  the  first  line  of  the  eyepiece 
micrometer,  being  careful  to  leave  all  the  con- 
ditions unchanged.  Note  how  many  divisions  are 
required  to  cover  the  object  and  divide  by  the 
figure  first  obtained  with  the  stage  micrometer. 
Thus,  if  there  are  40  spaces  which  we  know  are 
0.1  mm.  divisions  the  real  size  of  the  object  will  be 
0.5  mm.,  or  forty  tenths  divided  by  eight. 

If  measurements  are  made  under  exactly  the 
same  conditions  of  tube  length,  with  same  objec- 
tives, it  will  be  unnecessary  to  repeat  the  opera- 
tion with  eyepiece  and  stage  micrometer,  as  the 
proper  ratio  may  be  marked  on  a  card,  as  it 
remains  constant. 

The  most  efficient  apparatus  however,  for  obtain- 
ing accurate  results  is  with  the  filar  or  screw  micro- 
meter (Fig.  46).  This  consists  of  a  metal  case  to 
the  upper  surface  of  which  is  fitted  an  adjustable 
Ramsden  eyepiece.  Within  is  a  frame  carrying 


174 


one   or  several   delicate  spider    lines,    which    are 
moved   across   the   space  to   be  measured   by  an 


Fig.  46. 

accurate  screw  of  either  0.5  mm.  or  -fa  inch  pitch, 
which  has  at  its  end  a  graduated  disk  divided  in 
100  or  finer  spaces,  thus  giving  a  definite  value  for 


Field  of  Large  Filar  Micrometer 
showing  cross  hairs  and  recording  comb. 

each  space.     An    adapter    is    also    provided    for 
attaching  to  the  tube  of  microscope. 


TO  SELECT  A  MICROSCOPE. 


When  a  person  has  concluded  to  obtain  a  micro- 
scope, a  suitable  selection  is  a  matter  of  consider- 
able importance  to  him.  The  varieties  are  in- 
numerable, prices  without  end,  all  sorts  of  claims 
made  for  them. 

The  variety  of  special  lines  of  investigation  in- 
volves nearly  as  great  a  variety  of  requirements. 
The  amount  of  money  to  be  expended  ;  what  shall 
be  the  stand ;  what  the  objectives  ;  shall  the 
entire  outfit  be  purchased  at  one  time  or  little  by 
little  ;  are  all  questions  of  paramount  importance, 
which  the  writer  does  not  expect  to  solve,  but 
hopes  to  give  sufficient  information  so  that  a  more 
intelligent  selection  may  be  made  than  might 
probably  be  done  otherwise. 

If  one  has  a  friend  or  teacher,  who  is  generally 
accepted  as  an  authority,  it  will  be  well  to  consult 
him  or  her  and  obtain  suggestions  as  to  the  most 
suitable  selection  for  the  intended  work  and  such 
advice  will  always  be  gladly  given.  Or,  if  advice 
is  asked  of  a  reputable  firm,  the  writer  is  convinced 
that  it  will  be  honestly  and  disinterestedly  given. 


176 

When  means  will  permit,  the  outfit  for  im- 
mediate requirements  should  be  obtained  complete 
and  as  Prof.  Gage  says,  "  the  best  that  can  be 
afforded  should  be  obtained,"  and  further,  "  even 
when  all  the  optical  parts  cannot  be  obtained  in 
the  beginning  it  is  wise  to  secure  a  stand  upon 
which  they  may  all  be  used  when  they  are  finally 
-secured."  The  writer  agrees  entirely  with  this 
advice.  Even  though  the  stand  be  plain,  it  should 
be  good,  with  the  necessary  adjustments  and 
capable  of  receiving  and  fully  utilizing  such  further 
accessories  as  may  be  obtained  later  on. 

Stand.  American  or  Continental  ? — First  of 
all,  choice  will  have  to  be  made  between  the  two 
types  and  while  one's  sense  of  the  aesthetic  may  be 
a  factor  it  is  mainly  the  practical  utility  which 
must  govern  the  decision.  Whether  large  or  small 
must  largely  be  determined  by  the  future  use  to 
which  it  is  to  be  put.  One  rule  may  apply  to  all" 
however,  and  that  is,  that  the  instrument. shall  be 
so  balanced,  that  it  will  be  absolutely  steady  dur- 
ing" manipulation  in  the  upright  or  inclined  position. 
In  general  the  low  stand  is  preferred  as  it  permits 
of  resting  the  arms  upon  the  table  while  mc>y;ng 
the  object  and  the  comfort  of  looking  through  the 
tube  whether  the  instrument  be  upright  or  in- 
clined. 

Tube  Length. — In  the  matter  of  tube  length 
the  optical  results  are  the  same  in  both,  so  that  a 


177 

conclusion  must  be  reached  in  so  far  as  it  effects 
the  height  of  the  instrument. 

Base. — The  base  is  an  important  feature  and 
while  not  over  heavy  should  insure  steadiness  by 
the  proper  form  and  disposition  of  metal ;  it 
should  not  rest  on  more  than  three  points,  with  the 
rear  one  fairly  distant  from  the  pillar. 

The  Joint  for  Inclination  of  Arm. — This, 
without  question  is  an  advantage  and  while  it  is 
an  inexpensive  addition  it  will  add  considerably  to 
the  comfort  of  working  and  should  invariably  be 
present,  if  pecuniary  means  do  not  absolutely 
prohibit  it. 

Coarse  Adjustment.— Almost  all  instruments 
for  reliable  work  are  provided  with  both  fine  and 
coarse  adjustments.  The  choice  of  the  latter  lies 
between  the  sliding  tube  or  rack  and  pinion.  The 
former  has  only  the  advantage  of  economy  and  is 
a  decided  disadvantage  in  the  hands  of  students  in . 
injuring  objectives  and  preparations.  Further 
than  this,  it  is  almost  impossible  for  the  maker  to 
center  the  nosepiece  with  the  tube,  so  that  a  change 
of  objective  usually  throws  an  object  out  of  the 
field  and  requires  that  it  be  looked  for  anew  with 
each  change.  With  the  rack  and  pinion  the  nose- 
piece  has  an  unvarying  relation  to  the  tube  and  is 
not  liable  to  this  difficulty  and  offers  a  steady  and 
agreeable  adjustment.  The  advantages  of  the  rack 


178 

and  pinion  seem  to  be  generally  appreciated  in  this 
country  and  there  are  few  instruments  sold  and 
used  without  it.  Dr.  Stokes  speaks  of  the  sliding 
tube  adjustment  as  follows  : 

"  This  is  a  very  inconvenient  and  undesirable 
arrangement.  It  is  awkward,  since  the  friction  is 
often  so  great  that  the  whole  stand  will  move  out 
of  position  before  the  body  will  budge,  and  fre- 
quently, more  frequently  than  not,  even  when  the 
foot  is  heavy  enough  to  keep  the  instrument  firmly 
on  the  table,  both  hands  are  needed  to  manipulate 
the  body.  It  is  dangerous  too,  since  under  circum- 
stances, the  body  has  the  obnoxious  habit  of  sud- 
denly slipping  further  than  the  microscopist  in- 
tends, stopping  only  when  it  crashes  against  the 
slide,  where  it  usually  grinds  and  crunches  cover 
glass  and  objective  with  apparently  fiendish  glee. 
A  stand  without  a  coarse  adjustment  by  rack  and 
pinion  is  a  good  stand  to  be  permanently  left  with 
the  optician.  No  fine  microscopical  work  can  be 
done  with  an  instrument  whose  body  slides  through 
a  friction  collar.  That  arrangement  may  be  cheap, 
but  it  is  also  a  torment  and  a  peril." 

Rack  and  Pinion. — This  should  be  absolutely 
smooth  with  no  back-lash  or  lost  motion  through- 
out its  entire  length,  which  can  be  determined  by 
holding  the  main  tube  and  working  the  pinion 
buttons  very  slightly  but  quickly  back  and  forth. 
It  should  be  perfectly  fitted  in  its  bearings,  so  tha.t 


179 

there  will  not  be  the  least  side  motion  and  this 
should  be  tested  under  the  magnifying  power  of 
an  objective.  There  should  be  no  sensation  of  the 
individual  teeth  coming  in  contact.  ,  It  is  safe  to 
assume  that  if  the  rack  and  pinion  shows  either  of 
the  above  defects,  the  instrument  is  faulty  in  other 
directions  as  well. 

Fine  Adjustment. — Nothing  in  the  microscope 
will  cause  more  aggravation  than  a  faulty  fine 
adjustment.  It  should  work  absolutely  smooth 
with  no  side  play  in  the  screw.  The  body  should 
respond  promptly,  when  moving  the  milled  head 
rapidly  forward  and  backward  and  should  not 
cause  any  swaying  of  the  image  during  observa- 
tion. The  micrometer  screw  should  be  back  of  the 
pinion,  not  at  the  front  of  the  ttibe  nor  under  the 
stage. 

Metal. — Whether  an  instrument  shall  be  of 
japanned  iron  or  lacquered  brass  is  probably 
largely  determined  by  the  amount  of  money  to  be 
expended.  So  far  as  the  intrinsic  suitability  of 
the  metals  is  concerned  there  is  no  difference. 
Brass  however,  offers  a  maker  a  better  opportunity 
tor  displaying  his  mechanical  skill  and  while  it  is 
no  doubt  true,  that  many  highly  finished  instru- 
ments are  of  poor  workmanship  in  their  working 
parts,  it  is  also  a  fact  that  a  well  made  instrument 
is  always  nicely  finished. 


180 

Size  and  Weight. — The  size  of  instrument  is 
worthy  of  consideration.  If  an  instrument  is  to 
remain  stationary  in  a  practitioner's  office  or  labo- 
ratory, it  may  be  large  without  being  cumbersome. 
If,  however,  it  is  intended  to  be  carried  about  it 
should  be  of  the  smaller  and  more  contracted 
pattern. 

Working  Space  Below  Stage. — Another  im- 
portant consideration  is  the  space  between  the 
stage  and  base,  or  table.  While  it  is  advisable  to 
have  the  stage  low  on  account  of  the  convenience 
in  manipulating  a  slide,  there  should  still  be  suf- 
ficient space  for  the  convenient  attachment  of  sub- 
stage  accessories.  In  this  respect  the  American 
instruments,  whether  of  the  American  or  Conti- 
nental type,  are  superior  as  they  are  built  for  the 
better  accommodation  of  accessories. 

Stage. — A  variety  of  stages  is  offered  on  in- 
struments of  similar  construction.  The  plain,  flat 
stage  while  preferred  by  some,  offers  no  advan- 
tages over  the  ordinary  round  one,  unless  specially 
made  for  examining  specimens  on  larger  slides  than 
the  standard  3  by  1  inch.  Those  stages,  covered 
on  top  with  vulcanite,  offer  many  advantages. 
The  spring  clips  are  usually  of  similar  construction, 
although  varying  in  detail  and  curves.  Properly 
constructed  clips  should  have  such  thickness  of 


181 

metal  and  be  so  bent  as  to  allow  specimens  to  be 
brought  under  them  without  resistance  and  keep 
them  properly  in  place,  without  too  much  pressure 
and  consequent  friction. 

A  Glass  Stage  and  Slide  Carrier  may  be  con- 
sidered a  good  investment,  as  it  admits  of  the 
convenient  manipulation  of  the  slide,  without  the 
grating  feeling  which  usually  accompanies  the 
direct  movement  of  the  slide  on  the  stage. 

The  Mechanical  Stage,  while  an  absolute 
necessity  in  petrographical  and  other  work  where 
a  systematic  search,  as  for  bacilli  or  in  blood  count- 
ing, over  the  entire  surface  of  object  is  required,  it 
will  also  be  found  a  most  useful  accessory.  The 
obstacle  of  considerable  cost  which  prevailed  until 
the  present  time,  is  now  removed  and  good 
mechanical  stages  may  be  obtained  at  a  very 
reasonable  cost.  They  are  supplied  in  two  forms, 

Fixed  mechanical  stage,  in  which  the  mechanical 
movement  is  built  on  a  stage. 

Attachable  mechanical  stage,  which  can  be  at- 
tached to  the  Continental  stands  having  plain 
stage.  This  has  advantages,  since  it  may  be  re- 
moved, leaving  the  stage  plate  free  but,  of  course, 
cannot  be  revolved.  Either  form  involves  the 
most  delicate  work  and  while  the  parts  are  neces- 
sarily small  should  be  built  with  a  view  to  strength 
and  durability.  They  should  work  with  the 


182 

utmost  precision   and  smoothness   and   with  abso- 
lutely no  lost  motion. 

Revolving  Stage. — This  is  also  a  great  con- 
venience in  all  work,  while  being  a  necessity  in 
some  directions,  and  when  provided  with  centering 
screws  may  be  used  to  some  extent  as  a  mechani- 
cal stage  with  only  a  limited  movement  however. 
It  should  work  freely  with  the  rolling  motion  of 
one  finger,  without  any  side  play  and  without 
throwing  the  object  out  of  the  plane  of  focus 
during  revolution. 

Substage. — This  is  an  absolute  necessity  in  a 
modern  microscope,  except  perhaps  for  students 
use  in  primary  work.  It  should  have  a  vertical 
adjustment  and  preferably  with  rack  and  pinion. 
If  possible  select  the  complete  substage  attachment. 

Substage  Condenser. — If  means  will  permit, 
purchase  this,  as  it  is  in  all  work  most  convenient 
and  in  some  directions,  like  bacteriology,  abso- 
lutely necessary.  The  Abbe  condenser  is  the 
cheapest  form  giving  good  results  and  one  with 
numerical  aperture  of  1.20  is  sufficient  in  all  cases 
unless  oil  immersion  objectives  of  the  greatest 
aperture  are  used. 

Objectives  and  Eyepieces. — It  is  hoped  that 
the  information  given  of  the  various  qualities  in 
an  objective  will  aid  to  make  a  suitable  selection  of 


is-, 

the  optical  parts.  Since  the  stands  have  been 
classified  as  of  long  and  short  standard  tube 
lengths,  the  first  quality  to  look  for  is,  after  the 
stand  has  been  selected,  the  suitability  of  objec- 
tive and  eyepiece  to  it  and  to  the  work.  As  a 
variety  of  powers  is  obtained  by  a  suitable  combi- 
nation of  eyepieces  and  objectives  and  while  power 
in  itself  can  be  obtained  by  increasing  the  power 
of  eyepiece,  this  is  not  advantageous  as  we  have 
shown. 

Eyepiece. — Select  the  Huyghenian  eyepiece 
and  of  no  higher  power  than  f  inch.  In  cata- 
logues many  outfits  are  made  up  of  one  eyepiece 
and  two  objectives,  but  this  is  only  for  economy  ; 
it  is  always  advisable  to  select  two  eyepieces,  per- 
ferably  the  2  inch  and  1  inch  and  insist  that  they 
be  parfocal,  as  this  will  be  found  extremely  con- 
venient and  will  not  disturb  the  optical  standard 
length.  If  for  any  work  \  inch  or  higher  powers 
are  desired,  the  solid  eyepiece  may  be  recom- 
mended. With  the  apochromatic  objectives  use 
the  compensating  eyepiece  only.  Every  eyepiece 
should  be  marked  with  its  equivalent  focus. 

Objectives. — For  all  ordinary  student  and  pro- 
fessional work,  not  involving  bacteriological  inves- 
tigations, the  |  inch  0.24  N.  A.,  and  \  inch  0.62 
N.  A.  for  long  tube  ;  and  f  inch  0.25  N.  A.,  and  1 
inch  0.82  N.  A.  or  \  inch  0.85  N.  A.,  for  short  tube 
have  generally  been  accepted  as  best  suited. 


184 

Bacteriological  investigations  absolutely  require 
an  oil  immersion  objective  in  which  the  T^  inch  of 
1.32  N.  A.  is  generally  employed. 

Botanical  work  necessitates  a  2  inch  or  3  inch  in 
addition  to  the  regular  outfit. 

Objectives  of  Wide  Aperture.— It  will  be 
noted  that  objectives  of  the  lowest  price  and  lowest 
in  the  scale  of  efficiency  have  been  recommended 
as  ample  for  ordinary  use,  but  it  is  well  to  bear  in 
mind  or  study  the  advantage,  which  is  obtained  by 
objectives  of  larger  angular  aperture.  These 
advantages  are  absolute  and  unquestionable,  but 
whether  commensurate  with  the  additional  pecu- 
niary outlay,  must  be  left  mostly  to  the  judgment 
of  the  purchaser.  That  he  may  be  somewhat 
guided,  we  may  say  that  the  selection  of  higher  or 
highest  grade  objectives  is  not  by  any  means 
exceptional,  but  general,  and  would  undoubtedly 
be  more  common  but  for  the  barrier  of  expense. 

In  these  days  of  competition,  prices  alone  are 
too  often  made  the  object  of  inducement,  without 
any  reference  to  quality.  Be  distrustful  of  all 
such  objectives  and  if  contemplating  their  pur- 
chase, always  reserve  the  right  of  having  them 
examined  by  an  expert.  Have  a  distrust  especially 
of  all  "  nameless  "  objectives.  It  is  safe  to  assume 
that  if  the  maker  cannot  attach  his  name  he  is 
doubtful  of  their  quality. 


185 

It  is  sometimes  found  that  dealers  offer  the  same 
objectives  of  different  quality  at  different  prices. 
Too  great  care  cannot  be  observed  in  such  cases, 
as  the  very  fact  of  the  admission  of  a  difference  in 
quality  indicates  that  they  are  made  by  an  unre- 
liable maker.  This  mode  of  offering  objectives 
was  in  vogue  many  years  ago  when  the  principles 
of  optics  and  facilities  for  making  were  limited  and 
when  a  higher  price  was  asked  for  those  which 
might  be  termed  a  happy  combination.  There  is 
no  excuse  however,  at  the  present  day,  for  any- 
thing of  this  kind,  because  every  conscientious 
optician  has  his  standard  for  every  objective. 

In  purchasing  a  microscope  a  beginner  may  be 
easily  misled  by  the  enticing  appearance  of  an 
object,  which  may  be  due  not  so  much  to  the  in,- 
strument  as  to  the  object  itself  and  if  the  optical 
parts  are  inferior,  it  will  require  but  a  short  ex- 
perience to  become  convinced  of  it — usually  as 
soon  as  a  comparison  can  be  made  with  reliable 
work.  The  investment  in  one  of  these  objectives 
is  not  only  a  source  of  disappointment,  but  usually 
proves  to  be  a  •  pecuniary  loss,  as  it  is  generally 
followed  by  a  fresh  outlay  in  responsible  work. 

It  is  of  ordinary  occurrence  that  such  objectives 
as  have  been  spoken  of  are  sent  to  the  writer's 
firm  with  the  request  to  examine  them  and  rectify 
the  faults  ;  but  an  examination  almost  invariably 
proves  that  the  cost  of  doing  so  is  sonsiderably 


186 

greater  than  purchasing  a  new  objective  of  the 
same  power  and  it  would  not  even  then  be  equal 
to  the  latter. 

Accessories. — We  have  already  stated  in  the 
body  of  this  book  which  accessories  are  considered 
useful.  Some  of  them  are  absolutely  necessary  in 
some  special  lines  of  work,  in  which  case  however, 
the  student  is  generally  conversant  with  the  re- 
quirements and  may  make  a  suitable  selection, 
but  for  all  general  purposes  some  accessories  are 
necessities  where  others  are  only  conveniences 
and  we  append  a  list  of  such  which,  unless  pro- 
hibited by  necessity,  should  accompany  each  outfit. 

Abbe  substage  condenser,  preferably  the  com- 
plete substage  attachment  giving  all  adjustments. 

Double,  triple  or  quadruple  nosepieces  according 
to  the  number  of  objectives  accompanying  the 
microscope. 

Abbe  camera  lucida. 

Revolving  or  attachable  mechanical  stage. 

Eyepiece  and  stage  micrometer. 

Mounted  objects,  Proboscis  of  Blow-fly  and  P. 
angulatum,  dry. 

Pocket  magnifier,  preferably  Aplanatic  or  Hast- 
ings triplet. 

Cover  glass  gauge. 

Flat-wick  oil  lamp. 

Dissecting  stand  or  dissecting  microscope. 


187 

Besides  these  there  are  other  requirements  such 
as  slides,  covers,  mounting  media,  forceps,  etc.,  the 
necessity  of  which,  however,  can  be  better  deter- 
mined from  books  devoted  to  this  purpose.  There 
are  other  articles  which  in  some  directions  are 
necessities,  but  are  general  conveniences,  among 
which  may  be  mentioned  : 

Adjustable  drawing  table. 

Polariscope. 

Photomicrographic  camera. 

Live  box  or  compressors. 

Eye  shade. 

Turn  table. 

Revolving  microscopical  table. 

Cabinet  for  objects. 


CARE  OF  A  MICROSCOPE. 

Besides  acquiring  the  ability  to  properly  use  an 
instrument  with  its  accessories,  it  is  important  to 
know  how  to  keep  it  in  the  best  working-  condition. 
It  may  be  said  without  reserve  that  an  instrument 
properly  made  at  the  outset  and  judiciously  used, 
should  hardly  show  any  signs  of  wear  either  in 
appearance  or  in  its  working  parts,  even  after  the 
most  protracted  use  ;  and  further  than  this,  every 
good  instrument  should  have  a  provision  for  taking 
up  lost  motion,  if  there  is  a  likelihood  that  this 
may  occur  in  any  of  the  parts. 

Especial  care  should  be  given  to  the  optical 
parts,  in  fact  such  care,  that  they  will  remain  in  as 
good  condition  as  when  first  received.  Accidental 
injury  may  of  course  occur  to  them,  but  if  a  syste- 
matic manner  of  working  is  followed  and  a  special 
receptacle  for  each  part  is  provided,  this  may 
usually  be  avoided. 

To  Take  Care  of  a  Stand. — Keep  free  from 
dust  is  one  of  the*  first  rules  to  be  observed.  This 
may  be  done  in  a  manner  formerly  prescribed.  If 
dust  settles  on  any  part  of  the  instrument  remove 


189 

it  first  with  a  camel's  hair  brush  and  then  wipe 
carefully  with  a  chamois  skin,  wiping  with  the 
grain  of  the  finish  of  the  metal  and  not  across  it,  as 
in  the  latter  case  it  is  likely  to  cause  scratches. 
Keep  the  working  and  sliding  parts  absolutely 
free  from  dust,  as  this  grinds  and  will  thus  soon 
cause  play. 

Use  no  alcohol  on  any  part  of  the  instrument \  as 
it  will  remove  the  lacquer.  As  the  latter  is  for  the 
purpose  of  preventing  oxydization  of  the  metals,  it 
is  important  to  observe  this  rule. 

To  use  the  draw -tube  impart  a  spiral  motion. 

To  lubricate  any  of  the  parts,  use  a  slight  quan- 
tity of  soft  tallow  or  good  clock  oil,  or  pararfine  oil. 

If  the  pinion  ivorks  loose  from  the  jar  incident  to 
transportation  or  long  use,  which  sometimes  occurs 
to  such  an  extent  that  the  body  will  not  remain  in 
position,  increase  its  tension  by  tightening  the 
screws  on  pinion  cover. 

In  using  a  screw -driver,  grind  its  two  large  sur- 
faces so  that  they  are  parallel  and  not  wedge- 
shape  and  so  it  will  exactly  fit  in  the  slot  of  the 
screw-head. 

In  inclining  the  stand  always  grasp  it  by  the  arm 
and  never  by  the  tube,  as  in  the  latter  case  it  may 
loosen  the  slide  or  tear  off  some  of  the  parts. 

When  repairs  or  alterations  are  necessary,  always 
have  these  make  by  the  manufacturers,  who  can, 


190 

from  a  system  of  duplicated  parts,  not  only  do  it 
cheapest,  but  best. 

To  Take  Care  of  Objectives  and  Eyepieces.— 

The  utmost  cleanliness  must  be  observed  with 
objectives  and  eyepieces.  When  indistinct,  dark 
specks  show  in  the  field,  the  cause  may  usually  be 
looked  for  in  the  field  lens  of  the  eyepiece,  although 
sometimes  in  the  eye  lens  also.  The  dust  may  be 
removed  by  a  camel's  hair  brush,  but  when  this  is 
not  sufficient  use  a  well  washed  piece  of  linen,  such 
as  an  old  handkerchief.  From  its  fine  texture 
chamois  skin  is  desirable,  but  as  it  is  fatty  it  should 
never  be  used  until  after  it  has  been  well  washed. 

The  same  method  applies  to  cleaning  objectives. 
Clean  an  immerson  objective  immediately  after  it  has 
been  used,  first  by  removing  the  fluid  with  a  moist 
linen  and  then  by  iising  a  dry  piece,  or  by  means 
of  Japanese  lens  paper.  Never  separate  the  systems 
of  objectives,  even  if  they  can  be  unscrewed  by  the 
fingers  ;  it  is  always  dangerous  as  they  are  liable 
to  become  decentered. 

Keep  the  objectives  especially  in  a  place  where 
they  are  not  subject  to  extreme  and  sudden  changes 
of  temperature,  as  the  unequal  expansion  and  con- 
traction of  glass  and  metal  may  cause  the  cement 
between  the  lenses  to  crack.  Also  keep  them  from 
direct  sunlight. 


191 

Screw  them  into  the  nosepiece  and  unscrew,  by 
grasping  the  milled  edge. 

Avoid  any  violent  contact  of  the  front  lens  with 
the  cover  glass.  Usually  the  latter  suffers,  but  it 
is  as  liable  to  injure  the  former. 

Above  all,  the  owner  should  make  it  a  mle  that 
no  one  except  himself  handles  the  microscope  and 
accessories.  One  person  may  be  expert  in  the 
manipulation  of  one  instrument  and  still  find  it 
difficult  to  work  with  another.  The  fine  adjust- 
ment particularly  causes  the  greatest  difficulty,  as 
in  some  instruments  it  corresponds  with  the  move- 
ment of  the  micrometer  screw,  while  in  others  it  is 
contrary  and  thus  the  objective  as  well  as  the 
object  is  endangered. 


INDEX. 


ABBE   CAMERA   LUCIDA, .T^^iii ._....   164 

Abbe  Condenser, '154,  182 

Aberration,  Chromatic,  24,  25,  94,  142 

Aberration,  How  Corrected, 25,  26 

Aberration,  Spherical, 22,  23,  96,  140 

Accessories,   186 

Achromatic  Lens, 26 

Adjustments,  47 

Adjustable  Objectives, 98 

Adjustment,  Closed, 98 

Adjustment,  Open,  • 99 

American  Type  Microscope, 176 

Amplification, no 

Angular  Aperture, 81 

Apertometer,  91 

Aperture, 83,  87 

Aperture  of  Condenser,    160 

Aplanatic  Triplet,  30,  38 

Apochromatic  Objectives, 112 

Arm  of  Microscope,   45 

Attachable  Mechanical  Stage, 57,  181 

Axis  of  Lens,  19 

Axis,  Optical,   49 

BASE    OF    MICROSCOPE, 45,  67,  177 

Binocular  Microscope,   61 

Blood  Counting, 58 

Body  of  Microscope, 45,  61 

Bruecke  Lens, 31 

Burning  Point, 21 

CAMERA  LUCIDA, 163 

Camera  Lucida,  Abbe,   164 

Camera  Lucida,  Wollaston's, 164 

Cap  Diaphragm, 70 

Cedar:  Oil, 77 

Centering  Illumination, 158 


11)4 

Centering  Screws, 48,  55 

Chromatic  Aberration, 24,  25,  94,  142 

Clips, 48 

Closed  Adjustment,   98 

Coarse  Adjustment, 47,  62,   177 

Coddington  Lens,   29 

Collar, 47 

Collective  Lens,  74 

Colors,  Primary, 24 

Compensating  Ocular,  ..-..- 112,  117 

Compound  Microscope, 15,  42 

Concave  Lenses, 18,   19 

Condenser,  Abbe, 154,  182 

Condenser,  Achromatic, 155 

Condenser,  Aperture  of, 160 

Condenser,  Centering  of,  157 

Condenser,  Focusing  of, 159 

Condenser,  Purpose  of, 154 

Continental  Eyepiece 115 

Continental  Microscope, 50,  176 

Convex  Lenses, 18,  19 

Corrected  Lens,  26 

Cover  Glass, 49,  83,  96,  142 

Cover  Glass,  Corrections  for,  96-103 

Cover  Glass  Gauge, ; 101 

Cover  Glass,  Thickness  of, 96 

DIAMETERS 39 

Diaphragm,   24,  49,  70,   114 

Diatoms,  How  to  Resolve, 150,  151,  152 

Diatom  Test  Plate, 132 

Diatoms,  When  Resolved, 139 

Dispersive  Quality, 24 

Dissecting   Microscopes 28 

Dissecting  Microscope,  How  to  Use, 35 

Dome  Diaphragm, 70 

Double  Nose-piece 60 


19,-) 

Doublet So 

Drawing  Board,    166,   168 

Draw  Tube,   46,  66 

Dry  Objectives,   77,  85,  86,   144 

ENGLISH    EYEPIECE,   115 

Excelsior  Dissecting  Microscope, 35 

Eye,  Which  to  Use, 130 

Eye  Lens, 74 

Eyepiece, 46,   72,   112,   113,   182,   183 

Eyepiece,  Care  of 180 

Eyepiece,  Diaphragm  in 119 

Eyepiece  Micrometer, 171 

Eyepiece,  Parfocal, 119 

Eyepiece,  Rating  of 117 

Eyepiece,  to  Attach, 121 

FIELD,    FLATNESS   OF 104,   118 

Field  Lens 74 

Field  of  Objectives, uo,   118 

Filar  Micrometer, 173 

Fine  Adjustment,   47,  65,   179 

Flatness  of  Field 104,   118 

Flint  Glass,    27 

Focusing, 128 

Focal  Distance, 19 

Focal  Point,   19,  21 

Focus,  Real, 20 

Focus,  to  Determine, 20 

Focus,  Virtual 20 

GLASS   COVERS, 49,  83,  96,   142 

Glass,  Crown,   27 

Glass,  Flint, 27 

Glass,  Jena,   74 

Glass,  Reading, 31 

Glass  Stage    56 

Graduated  Head 65 

Graduated  Stage, 55 


196 

HASTINGS   TRIPLET, 30 

High  Power  Objectives, 79 

Homogeneous  Immersion  Fluid, 78 

Homogeneous  Immersion  Objectives, 85,  86,  147 

ILLUMINATION, 154,  158,  161,   124 

Image,  Magnified, 42 

Immersion   Fluid, 77 

Immersion  Objectives, 77,   147 

Index,  65 

Iris  Diaphragm. , 70 

JENA  GLASS, 74 

Joint  for  Inclination, 68,   177 

KELLNER  EYEPIECE, 116 

LENS,   ACHROMATIC, 26 

Lens,  Bruecke, 31 

Lens,  Coddington, 29 

Lens,  Collective, 74 

Lens,  Eye, 74 

Lens,  Field, 74 

Lens,   Hastings, 30,  38 

Lens  Stands  and  Holders, 32 

Lens,  Triplet, 30,  38 

Lenses, 15 

Lenses,  Convex, 18,   19 

Lenses,  Concave, )  8,   19 

Lenses,  Converging, 18 

Lenses,  Diverging, 18 

Lenses,  Meniscus,   18,   19 

Lenses,  Optical  Properties  of, 15 

Lenses,  Stopped  Down,   24 

Light,  Refraction  of, 16,   17 

Linear  Magnification, 39 

Linen  Tester 33 

Long  Tube  Microscopes, 50 

Low  Power  Objectives, 79 

MAGNIFICATION,  Linear,  In  Areas,  In  Diameters,    39 


19? 

Magnified  Image 42 

Magnifier,  Tripod, 32 

Magnifiers, 28 

Magnifiers,  How  to  use, 35 

Magnifying  Power, 21,  38,  109 

Magnifying  Power,  to  Determine, 40,  169 

Mechanical  Stage,  56,   181 

Medium  Power  Objectives, 79,   138 

Meniscus  Lenses, 18,   19 

Micrometer,  Eyepiece 171,   116 

Micrometer,  Filar, 173 

Micrometer  Screw, 65 

Micrometer,  Stage, 170 

Microscope,  American  Type, 176 

Microscope,   Binocular, 61 

Microscope,  Care  of, 188 

Microscope,  Dissecting,  How  to  use, 35 

Microscope,  How  to  Select, 175 

Microscope,  How  to  Work  with, 136 

Microscope,  Material  of, 179 

Microscope,  Parts  of, 45 

Microscope,  Purpose  of,  15 

Microscope,  Simple  and  Compound, 15,  28,  42 

Microscope,  Size  and  Weight, 180 

Microscope  Stand, 43 

Microscope,  To  Set  Up, 135 

Microscopes, 28 

Microscopes,  Classification  of, 49 

Microscopes,  Continental  Model,     50,   176 

Microscopes,  Jackson  Model,     49 

Microscopes,  Ross, 49 

Milled  Edge, 47 

Milled  Head, 47 

Mirror, 48,  69 

Mirror  Bar, 48,  69 

Mirror,   Focal  Point  of, 69 


19S 

Moellers  Test  Plate, 132 

Monocular  Microscope,   61 

Movable  Object  Carrier, 56 

NEGATIVE   EYEPIECE, u4 

Nose-piece,   ...  46,  59 

Numerical  Aperture, 87 

Numerical  Aperture,  How  to  Measure, 89 

OBJECT,    49 

Object  Carrier,  Movable, 56 

Object,  Opaque, 137 

Object,  Resolution  of, 92 

Object,  to  Find, 122 

Object,  to  Illuminate,   124 

Object,  to  Measure, 170 

Object,  When  Centered, 49 

Objects,  How  to  Draw, 163 

Objects,  Test, 132 

Objects,  Test,  Which  to  Use, . . 131 

Objectives 46,  72,  77,  85,  86,  138,  147,   182,  183,   184 

Objectives,  Adjustable, 98,   144 

Objectives,  Angle  of,    Si 

Objectives,  Apochromatic, 112 

Objectives,  Care  of, , 190 

Objectives,  Classification  of, 79 

Objectives,  Powers  of, 79 

Objectives,  Rating  of, 78 

Obje  .tives,  Systems  of, 79 

Objectives,  Testing  of, 95,  96,   149 

Objectives,  to  Attach, 121 

Objectives,  Tube  Length  of, 78 

Oblique  Illumination, 161 

Ocular, 46,  72,   112,   114,   182,   183 

'  Oculars,  Care  of, 190 

Oculars,  Rating  of, 117 

Oil  Immersion  Objectives, 77,   78,  85,  86,   147 

Opaque  Object 137 


199 

Opened  Adjustment, 97 

Optical  Axis, 49 

Orthoscopic  Eyepiece, 116 

Over  Correction, 95 

PARFOCAL  EYEPIECES, 119 

Penetration, 103 

Periscopic  Eyepiece, 116 

Pillar  of  Microscope, 45 

Pinion , 63 

Plankton  Work, 58 

Pleurosigma  Angulatum,  109,  131,  132,  133,  138,  141,  146,  150 

Positive  Eyepiece, 114 

Power,  Magnifying, 21,  38,   109 

Primary  Colors, 24 

Principal  Axis, 19 

Principal  Focus, 19 

Projection  Eyepiece, 117 

RACK  AND  PINION, 47,  63,  177,  178 

Radius  of  Lens, 19 

Ramsden  Eyepiece, 1 16 

Reading  Glass, 31 

Real  Focus, 20 

Real  Image, 22 

Refraction, 16,  17 

Resolving  Power, 91 

Revolving  Diaphragm, 70 

Revolving  Nosepiece, 60 

Revolving  Stage, 55 

SCREW  MICROMETER,   173,  65 

Screw,  Universal, 46 

Searcher  Eyepiece, 115 

Secondary  Colors, 94 

Short  Tube  Microscopes, 50 

Simple  Microscopes, 15,  28 

Slide,  Object, 49 

Sliding  Tube  Adjustment, 47,  177 


200 

Slips,  Glass, 49 

Slow  Motion,  65 

Society  Screw, 46 

Solid  Eyepiece, 115 

Spectrum, 24 

Spectrum,  Secondary, 94,  112 

Spherical  Aberration, 22,  96,  140 

Spring  Clips, 48 

Stage 47,  53,  180 

Stage,  Glass, 56,  181 

Stage,  Micrometer, 170 

Stage,  Revolving, 55,  182 

Stage,  Space  Below, , 1 80 

Stand, 43,   176 

Striae, 119 

Substage, 48,  182 

Substage  Bar, 48 

Substage  Condenser, 154,   182 

TESTING   OBJECTIVES, 95,  96,  149 

Test  Objects, 132 

Test  Objects,  Which  to  Use 132 

Test  Plate, 132 

Triple  Nose-piece, . . . 60 

Triplet,  Hastings,  ....    30,  38 

Triplet  Lens, 30,  38 

Tripod  Magnifier, 32 

Triplet  System, ._ 80 

Tube  Length, 50,  78,   176 

UNDER-CORRECTION, 95 

Universal  Screw, 46 

VIRTUAL   FOCUS, 20 

Virtual  Image, 22 

WOLLASTON   CAMERA   LUCIDA, 164 

Water  Immersion  Objective, 85,  86 

Working  Distance, 106 

Working  Distance,  to  Measure, 107 


The  BAUSCH  &  LOME 

OPTICAL  COMPANY 

Issue  the  following-  Catalogues: 

MICROSCOPES,  MICROTOMES  AND 
LABORATORY  SUPPLIES. 

^t 

PHOTOGRAPHIC  LENSES  AND 
SHUTTERS. 

^t 

EYEGLASSES  AND  LENSES,  ETC. 

at 

Offices: 

ROCHESTER,  N.  Y., 
NEW  YORK  CITY,         CHICAGO,  ILL., 

130  Fulton  Street.  408  Masonic  Temple. 


Bauscb  $  Comb 

Optical  Company, 

manufacturers  of 

Microscopes,  Microtomes, 

Photomicrographic  Cameras,  Microscopic 

Accessories,  Centrifuges. 

^ 
Sterilizers,  Incubators,  Autoclavs,  Etc. 

j* 

Photographic  Lenses, 

Photographic  Shutters,  Etc.,  Etc., 

<* 

Offices: 

ROCHESTER,  N.  Y., 
NEW   YORK   CITY,       CHICAGO,   ILL., 

J30  Fulton  Street.  408  Masonic  Temple. 


UNIVEESITY  OF  CALIFOENIA  LIBEAEY, 
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